Biology and Health Sciences

Key unit competency 

To be able to identify symptoms of common infectious diseases and their prevention and treatment.

Learning objectives 

After studying this unit, I should be able to: 

•  Define pathogen, transmissible disease and host. 

•  Explain how the body’s defence mechanism can be enhanced by vaccination. 

• Explain the symptoms, prevention and control of malaria, Ebola and HIV and AIDS. 

•  Apply practices of hygiene and sanitation to control the spread of diseases. 

• Organise community campaigns on prevention and treatment of diseases.

Introduction 

Infectious diseases are also known as communicable diseases. They are diseases that are transmitted from one person to another. These diseases are usually caused by harmful micro-organisms. Examples of micro-organisms include viruses, fungi, protozoa and bacteria.  Look at the picture below. Can you tell what will happen to the other person who is not coughing?

Can the action by the person in the picture lead to transfer of disease or not? If so, can all diseases that we know be transmitted in such a manner? Why? What does this tell you about the diseases that you will learn about in this unit? 

12.1 Transmission of infectious diseases 

Infectious diseases are capable of spreading from one person to another either directly or indirectly.

Research Activity 

1.  Using text books, handouts and internet, research on the following terminologies: 

2.  How does the body defend itself against diseases. 

3. Write your findings in your notebook.

Pathogen: This is any disease-causing microorganism. Bacteria, protozoa and viruses that cause disease are called pathogens. Pathogens are also known as germs. 

Transmissible disease: These are diseases capable of spreading from one person to another, for example, common cold.

Host: This is an organism that harbours a disease-causing micro-organism, for example, water snails are hosts to schistosomes that causes bilharzia. 

Vector: This is a living organism that can transmit infectious diseases between human beings or from animals to human beings. Female anopheles mosquito is a vector to the plasmodium parasite malaria. 

Carrier: This is a person who has been infected but develops no signs or symptoms. This person is capable of transmitting the disease to other people. 

Signs: These are visible expressions brought about by a disease, for example, red spots on the body.

Symptoms: A physical or mental feature that is regarded as indicating a condition of disease, particularly such a feature that is felt by the patient, for example, nausea, dizziness and headache.  

Epidemiology: The study of all factors that contribute to the appearance of a particular disease.

Activity 12.1 Visit to a health centre 

1. Prepare a questionnaire by formulating a series of questions to ask a health worker about transmission of infectious diseases.  

You may include the following questions. 

2.  Hand over in your research to the teacher for assessment.

Infections can be spread in several different ways. Some of them are listed below: 

1.  Diseases that are transmitted through the air are referred to as air-borne diseases. Tuberculosis and common cold are examples. 

2.  Some diseases are transmitted through the consumption of contaminated water and food. They are referred to as waterborne or foodborne diseases respectively. Examples are cholera and typhoid fever. 

3.  Through blood transfusion or contact with contaminated blood. Examples of diseases that can be spread through contaminated blood are HIV and AIDS and malaria.

4.  Physical contact with an infected person can transmit a disease. These infectious diseases are termed as contagious. They include ringworms, scabies and Ebola. 

5.  Communicable diseases are also spread by vectors. They are therefore known as vector-borne diseases,  for example, sleeping sickness is spread by tsetse flies, malaria is spread by female anopheles mosquito. Yellow fever on the other hand is spread by black fly and some species of mosquitoes. 

6.  Some infections can also spread through unprotected sexual intercourse with an infected person. These diseases are known as sexual transmitted infections (STIs), for example, HIV and AIDS. 

7.  Germs can also spread from a mother to her unborn child, usually through blood or other body fluids. They can also pass through contact with skin or mucous membranes during delivery.

Self-evaluation Test 12.1 

1. Which one of the following is a vector for sleeping sickness? 

2.  If you came into contact with a person who was coughing, which diseases are you likely to catch? 

3.  Name two diseases transmitted through transfusion of contaminated blood.

12.2 Defense against infection 

Research  Activity 

1. Using text books and internet research on the ways by which the body defends itself from infections. 

2. Share your findings with other class members.

Immunity is the ability of the body to resist disease infection. This means a person can be exposed to factors that can cause a disease, yet they do not become sick. The body’s defense mechanism protects an organism from infections.

If pathogens do not encounter resistance from the body’s defense mechanism, almost all diseases would be fatal. Organisms must find a way of defending themselves against harmful micro-organisms like viruses, bacteria and fungi.

The body has many ways of defending itself from invasion by harmful microorganisms. In most cases the body’s defense mechanism prevents this from happening. Some of these defense mechanisms are made in a way to block pathogens from entering into the body. 

Other mechanisms eliminate pathogens that are already in the body. There are also defence mechanisms that attack micro-organisms when they persist inside the body. The ability of the body to prevent itself from diseases using its lines of defense is called resistance. 

Body defence mechanisms can either be first line or second line. 

(a) Body’s first line of defence 

This line of defence prevents harmful micro-organisms from entering the body.  It is the external defence system of the body. It includes both physical and chemical defence lines. The body does this using various barriers that intercept micro-organisms hence prevent them from entering the body.

Example of the body defence mechanism include: 

(i)  The skin creates a physical barrier that protects the cells inside the body against the entrance of pathogens. A healthy skin is rarely penetrated by pathogens. Certain chemical secretions produced by the skin helps to stop the growth of bacteria and fungi. 

(ii) The nose and passages leading to the lungs are lined with cells that produce sticky fluid called mucus that traps invading microbes and dust. Tiny hairs called cilia lines the trachea. They can move back and forth in a wave-like motion. Cilia trap microbes and dust particles and prevent them from entering the lungs. These particles then combine with mucus and are either coughed, sneezed out or swallowed and then passed out of the body in faeces. 

(iii) The stomach produces hydrochloric acid which destroys many of the microbes that enter the body in food and drinks we take. 

(iv) Tears act as a barrier to pathogens. Tears are a watery secretion produced by tear glands located in the outer eye. They contain a powerful enzyme that can digest and breakdown harmless substances. Therefore, harmful microorganisms cannot enter the body through the opening of the eye. 

(v)  Clotting of blood occurs when an open cut or wound exposes blood to air. Such a cut causes a break in the skin exposing the body to harmful micro-organisms. When a clot forms, it seals the opening and enables the wound to heal. 

(b) Body’s second line of defence 

When pathogens are able to get past the first line of defence, for example, through a cut in the skin, a disease develops. This prompts the second line of defence to become active. This happens through a sequence of steps called the immune response. The immune system attacks the pathogens. The body has a range of defence mechanisms that operate to prevent or destroy pathogens. These responses only react to the presence of any pathogen or foreign material.

White blood cells known as phagocytes are adapted to fight these disease causing micro-organisms in the following ways: 

(i) They move by amoeboid action. They send out cell extensions called pseudopodia which allow them to surround invading microbes and engulf them. 

(ii) Phagocytes release digestive enzymes which break down the trapped microbes before they can do any harm. This process is called phagocytosis. 

(iii) They also have the ability to change their shape. This enables them fit in the narrow capillaries hence reach the pathogens

Immunity 

This is the ability of an organism to resist a particular infection or toxin by the action of specific antibodies or some white blood cells. 

An antibody is a protein produced by the body’s immune system when it detects harmful substances, called antigens. Examples of antigens include micro-organisms such as bacteria, fungi, parasites, viruses, and chemicals. Each type of antibody is unique and defends the body against one specific type of antigen. 

Antibodies detect and neutralises the antigen by binding to it. This signals the production of other cells of the immune system to get rid of the invading microbes before it spreads or attacks the body. 

(a) Natural immunity 

Natural and artificial immunity 

Natural immunity comes from the body itself. It is divided into two: active and passive natural immunity.

(i) Active natural immunity may develop after recovering from a disease. The organism makes its own antibodies as a result of contact with antigen from disease-causing pathogen. Once the organism recovers from the disease, it can produce antibodies very quickly should the pathogens invade again. This makes the organism immune to the invading micro-organism. If someone gets a measles attack and gets healed, chances of the disease recurring is minimal. 

(ii) Passive natural immunity is immunity acquired by the foetus from the mother through the placenta and also through breast milk. This type of immunity is short-lived.

(b) Artificial immunity is obtained by introducing antigens into the body of an organism to protect the organism from a disease. It is divided into two: active and passive artificial immunity. 

(i)  Active artificial immunity is induced by introducing antigens into the body of an organism through the use of a vaccine. A vaccine contains antigens composed of living, dead or weakened pathogens. They are used to stimulate the body to recognise certain disease antigens and respond to them. Vaccines usually do not cause the disease.

(ii) Passive natural immunity is the transfer of immunity in the form of ready-made antibodies. This is immunity that comes from using antibodies produced in one organism to protect another organism from a specific disease. These antibodies are usually extracted from the serum (plasma without soluble proteins) of an animal that has recovered from the disease. Such immunity does not last long. 

12.3 Symptoms, prevention, control and treatment of common infectious diseases

a) Malaria 

Malaria is a protozoan disease caused by a parasite called Plasmodium. The parasites are spread through the bites of infected female Anopheles mosquitoes, which are the malaria vectors. There are several species of plasmodium parasite that cause malaria in human beings. 

They include P. falciparum and P. vivax. Malaria poses the greatest threat to human health.

Malaria affects some groups in the population more than it does to others. These groups are prone to developing a severe form of the disease, than others. They include infants, children under 5 years of age, pregnant women and patients suffering from HIV and AIDS. 

Symptoms of malaria 

(i)  High fever and periodic chills that may be accompanied with sweating. 

(ii)  Frontal headache. 

(iii) Pain in the joints. 

(iv)  General body weakness. 

(v) Lack of appetite and nausea that may be accompanied by vomiting. 

If malaria is not treated within 24 hours it can progress to severe illness, often leading to death. Children with severe malaria frequently develop severe anaemia and cerebral malaria. In adults, it may involve several malfunctioning of organs. 

Prevention and treatment of malaria 

Mosquito control is the main way to prevent and reduce malaria transmission. The following are some of the ways mosquitos can be controlled. 

(i) Clearing bushes around homes. 

(ii) Draining marshes, pools and stagnant water around homes. This destroys breeding places for mosquitos. 

(iii) Spraying light used oil on stagnant water that cannot be drained. Oil sprayed on stagnant water kills mosquito larvae by suffocating them. 

(iv) Spraying the inside walls of our houses with insecticides. 

(v) Closing doors and windows during the evening time to prevent mosquitoes from entering. Windows can also be screened with window proof wire mesh. 

(vi) Sleeping under a treated mosquito net.

(vii) Removing old tins, broken pots, polythene bags and other things that can hold water from the compound. This prevents mosquitoes from breeding.

(viii) Patients can be isolated during the infection period to prevent transmission to others. 

(ix) Fish-eating mosquitoes can be introduced into ponds and pools to feed on mosquito larvae and pupae. 

(x) Preventative medicines can be taken regularly but upon prescription  from a qualified medical doctor.

b) Cholera 

Cholera is an acute diarrhoeal infection caused by ingestion of food or water contaminated with the bacterium Vibrio cholerae. It has a short incubation period of 2 hours to 5 days, a factor that triggers its pattern of outbreaks. Cholera is endemic in poor countries and strikes areas with poor sanitary conditions and where drinking water is not well treated. Cholera affects both children and adults and can cause death within hours. 

Most people infected with cholera do not develop any symptoms. The bacterium is present in their faeces for 1-10 days after infection and is shed back into the environment, endangering the health of other people.

Most people who develop symptoms, have mild or moderate symptoms, while others develop acute watery diarrhoea with severe dehydration (lack of water in the body). This can lead to death if left untreated. 

Symptoms of cholera 

(i) Severe watery diarrhoea (rice water stool) and vomiting. 

(ii) General body weakness. 

(iii) Low blood pressure. 

(iv) The patient loses weight rapidly. 

(v) The patient develops wrinkled skin and sunken eyes because of dehydration.

Control and treatment of cholera 

(i)   Drinking water should be treated or boiled and stored properly. 

(ii)  Proper disposal of human wastes.  A well-managed sewage disposal system in towns and use of latrines. in the rural areas. 

(iii) Washing hands before and after eating and after visiting the toilet.

(iv) Covering cooked food to avoid contamination by house flies. 

(v)   Vaccination during epidemics or when one is visiting an infested area. 

(vi) Cholera is treated using antibiotics. Prevention of dehydration also helps in the cure.

c) HIV and AIDS 

The Human Immunodeficiency Virus (HIV) attacks the immune system. This weakens a person’s defence mechanism against infections and some types of cancer. As the virus destroys and impairs the function of immune cells, infected individuals gradually become immunedeficient. Immune function is typically measured by number of white blood cells in the body (CD4 cell count). 

Immunodeficiency results in increased susceptibility to a wide range of infections. These infections (opportunistic infections) are diseases that people with healthy immune systems can fight off. At this stage, the patient is said to be suffering from AIDS HIV and AIDS is transmitted from one person to another through various ways. They include: 

(i)   By birth, from an infected mother to her unborn baby. 

(ii)   By having unprotected sexual intercourse with an infected person. 

(iii) By transfusion of infected blood. 

(iv) By sharing of unsterilised surgical and skin piercing instruments with infected people. 

Symptoms of HIV and AIDS 

The symptoms depend on the stage of infection. People living with HIV and AIDS tend to be most infectious in the first few months. The first few weeks after initial infection, individuals may experience no symptoms or an influenza-like illness including fever, headache, rash or sore throat. 

As the infection progresses, the immune system weakens. An individual can develop other signs and symptoms, such as swollen lymph nodes, weight loss, fever, diarrhoea and cough. Without treatment, they could also develop severe illnesses such as tuberculosis, meningitis, and cancers such as lymphomas and Kaposi’s sarcoma, among others. 

Prevention and control 

The most effective way to prevent the spread of AIDS is through health and sex education. Everyone has to be enlightened on the risks of acquiring the virus through careless lifestyles. 

Patients of HIV and AIDS should take anti-retroviral drugs (ARVs) which prolong their life span. To prevent AIDS one also needs to avoid the ways through which it is transmitted. The following are the ways through which a person can prevent himself or herself from contracting the disease. 

d.)  Tuberculosis 

Tuberculosis (TB) is caused by bacteria called Mycobacterium tuberculosis that most often affects the lungs. TB is spread from person to person through the air. When a person with pulmonary TB coughs, sneezes or spits, they spread germs into the air. If a healthy person nearby inhales these germs they can become infected. 

People with low immune systems, such as people living with HIV and AIDS, malnutrition, diabetes, or people who smoke, have a much higher risk of being infected. 

Symptoms of pulmonary TB 

Common symptoms of active pulmonary TB are: 

(i)  Cough with sputum and blood at times 

(ii)  Chest pains 

(iii)  General weakness 

(iv)  Weight loss 

(v)  Fever 

(vi)  Night sweats 

When a person develops active tuberculosis, the symptoms; cough, fever, night sweats and weight loss may be mild for many months. This can lead to delays in seeking medical care, resulting in transmission of the bacteria to other people. 

Prevention and treatment of tuberculosis 

(i)  Isolate patients to prevent transmission of the disease to other people. 

(ii)  Immunise children with tuberculosis vaccine known as BCG. This may prevent the spread of tuberculosis. But the vaccine does not necessarily protect against pulmonary tuberculosis. 

(iii) Avoiding overcrowded places. Opening windows in overcrowded rooms to allow free circulation of air.

(iv) Drinking boiled milk. Some types of tuberculosis is transmitted from bovine. 

(v)  Once infected, tuberculosis is treated with antibiotics. Standard anti-TB drugs have been used for some time, and resistance to the medicines is widespread.  

e)  Ebola 

Activity 12.3: Watching a film about ebola 

You will observe videos showing doctors treating ebola patients. Observe the protective gear worn by the doctors.

Study questions 

(a) What causes Ebola? 

(b) How is Ebola transmitted to the human population? 

(c) What are some of the signs and symptoms of the Ebola virus disease? 

(d) What can you do, to prevent contracting the Ebola disease? 

(e) From the video you have watched, why are doctors in protective gear while handling the patients? 

(f) Why do you think the ebola patients are in isolated wards? 

The Ebola virus causes an acute, serious illness which usually leads to death if untreated. Ebola virus is introduced into the human body through contact with body fluids of infected animals such as chimpanzees, gorillas, fruit bats, monkeys, forest antelope and porcupines found ill or dead in the forest. 

Ebola then spreads through humanto-human transmission via direct contact (through broken skin or mucous membranes) with the blood, secretions, organs or other bodily fluids of infected people and with surfaces and materials such as bedding and clothing contaminated with these fluids. 

Medical workers too, become infected while caring for patients suffering from ebola. Burial ceremonies in which mourners have direct contact with the body of the dead patient can also play a role in the transmission of Ebola. People remain infectious as long as their blood contains the virus.

Symptoms of Ebola virus disease 

The incubation period; the time interval from infection with the virus to onset of symptoms is 2 to 21 days. Human beings are not infectious until they develop symptoms. 

(i)  First symptoms are the sudden onset of fever, fatigue, muscle pain, headache and sore throat. 

(ii) This is followed by vomiting, diarrhoea, rash, symptoms of impaired kidney and liver function.

(iii)  Internal and external bleeding with blood oozing from the gums, ears and eyes. 

Prevention of Ebola 

(i) Avoid direct contact with wild animals like bats and chimpanzees. 

(ii) Avoid direct contact with people showing signs of Ebola virus. 

(iii) Use gloves and other protective clothing when handling Ebola patients. 

(iv) Wash your hands regularly with soap and water. 

(v) Couples who have just recovered from the virus should abstain from sexual intercourse or use condoms. 

(vi) Burial of the dead patients should be done by health workers. 

f)  Salmonellosis 

This is an infection caused by Salmonella bacteria. The elderly, infants, and those with impaired immune systems are more likely to develop severe form of illness. 

Salmonella infection usually results from ingestion of the bacteria from contaminated food or water. Eggs, milk, meat or poultry are particularly high risk foods. Fruit and vegetables may also be contaminated, especially if manure has been used as fertiliser. People may become infected if they transfer animal faeces containing Salmonella bacteria from their hands to their mouths, for example, eating after touching animals and failing to wash hands.

Person-to-person spread may occur when hands, objects or food become contaminated. In most cases, the illness lasts for between 4 to 7 days. Most people recover without treatment. In some cases, the diarrhea may be so severe that the patient becomes dangerously dehydrated and must be hospitalised. 

Symptoms of salmonellosis 

Most people infected with salmonella experience diarrhoea, chills with fever, nausea with or without vomiting, headache, blood in the stool and abdominal cramps 12 to 72 hours.

Prevention and treatment of salmonellosis 

(i) People with salmonella infection should not be allowed to handle food or take care of children. 

(ii)  Food should be well cooked. 

(iii) Do not purchase dirty or cracked eggs. 

(iv)  Boil milk thoroughly before drinking. 

(v) Always wash your hands with soap and clean water. 

(vi) Wash fruits (including melons) and vegetables before eating. 

(vii) Swimming pools should be disinfected regularly. 

(viii) Recovery from Salmonella infection usually occurs within a week and antibiotic treatment is not normally required. 

g)  Typhoid 

Typhoid fever is a bacterial infection. It is caused by a bacterium called Salmonella typhi, which only lives in human. It is related to the bacteria that cause salmonella food poisoning. It can spread throughout the body, affecting many organs. Without prompt treatment, it can cause serious complications and can be fatal.

Typhoid fever is highly contagious.  A small number of people (carriers) recover from typhoid fever but continue to have the bacteria. Both ill people and carriers shed the bacteria in their faeces and urine. The bacteria is spread by eating food or drinking water contaminated with faeces or urine of patients and carriers. 

Typhoid is common in areas where hygiene standards are poor and water is likely to be contaminated with sewage waste.

Symptoms of typhoid 

(i)   Sustained high fever 

(ii)   Continuous headache 

(iii)  Malaise (feeling of being unwell) 

(iv)  Decreased appetite 

(v)  Enlarged spleen which may cause abdominal discomfort 

(vi)  Dry cough in the early stage of the illness 

(vii)  A flat rose-coloured rash which may be visible on the trunk (vii) Constipation or diarrhoea – constipation occurs more often than diarrhoea in adults 

(viii) Illness varies from mild with lowgrade fever, to severe with multiple complications

Prevention and treatment of typhoid 

(i)  Isolation of the patients to avoid spread of the disease by contact and sterilising clothes used by patients. 

(ii)  Food handlers in institutions, like hospitals, schools and restaurants should go for regular medical checkups and treatment if found infected. 

(iii)  Proper disposal of faeces in toilet and in pit latrines.  

(iv)  Water treatment and purification to kill the bacteria. 

(v)  Boiling drinking water and cooking of food properly to kill the bacteria. 

(vi)  Practice good hygienic methods like washing hands properly after visiting the toilet. 

(vii) Washing fruits and vegetables with clean water before eating. 

(viii) Vaccinations in the case of outbreaks of the disease in epidemics. 

(ix) Seek proper treatment from qualified medical doctor in case of an infection. Antibiotic treatment helps reduce the time a person is infectious. 

Self-evaluation Test 12.3 

1. Which disease among the following is the most highly contagious? 

2.  What is the simplest thing you can do to protect yourself from contracting a disease? 

3.  State ways you can use to sensitize your community about infectious diseases?

Unit summary 

• Pathogen is a bacterium, virus, or other micro-organism that can cause disease. 

• Infectious diseases are transmitted through: air, contact, blood or other body fluids, for example, urine, saliva, breastmilk, semen and vaginal secretions. 

• First line of defence is a combination of physical and chemical barriers that prevent all types of foreign agents from penetrating the outer layer of the body. It includes the skin, mucous membranes, hair and cilia, gastric juices, tears, sweat and saliva. 

• The second line of defence is a group of cells, tissues and organs that work together to protect the body. This is the immune system. 

• Immunity is the ability of an organism to resist a particular infection or toxin by the action of specific antibodies or sensitised white blood cells. 

• Vaccination is the injection of a killed or muted microbe in order to stimulate the immune system against the microbe, thereby preventing disease. 

• Some unhygienic human activities, such as improper disposal of sewage and other wastes, cause diseases. These diseases include cholera, typhoid and salmonella. 

• Malaria is caused by a protozoan parasite that invades the red blood cells. The parasite is transmitted by mosquitoes in many tropical and subtropical regions.

• Ebola is an infectious and generally fatal disease marked by fever and severe internal bleeding, spread through contact with infected body fluids. 

• Acquired immune deficiency syndrome (AIDS) is caused by the human immunodeficiency virus (HIV). It damages the immune system and interferes with the body’s ability to fight the organisms that cause disease. HIV is a sexually transmitted infection.