Biology

Key Unit Competence
Describe the social factors that affect good health and apply knowledge gained in familiar and unfamiliar contexts.

Learning objectives
–– By the end of this unit, I should be able to:
–– Explain what is meant by health and disease.
–– Identify different categories of disease and give an example of each.
–– Explain the theory of the disease and the causes, sources, transmission, symptoms and controls of the disease.
–– Discuss how global patterns of disease are studied.
–– Analyze and interpret records from a given hospital to identify diseases as
endemic, epidemic or pandemic.
–– Apply knowledge gained to classify common diseases.
–– Appreciate the importance of germ theory of disease by showing that the
death rate related to infections is greater than those caused by accidents.

Introductory activity
a. Suggest measures to be taken for addressing issues related to eating without washing hands.
b. Discuss on different communicable diseases got from eating without washing hands.

15.1 Germ theory of diseases
Activity 15.1
Discuss the following questions
a. What are the causes of death?
b. Why it is difficult to eradicate malaria in Rwanda?

The germ theory states that many diseases are caused by the presence and actions of specific microorganisms within the body. In 1677, Antoni van Leeuwenhoek was the first to observe microorganisms in the droplets of water. but he did not make the connection with disease. Later, Spallanzani and Louis Pasteur observed germs in the blood of people suffering from disease. They suggested that the germs were an effect of the disease rather than the cause. The observations and actions of Ignaz Semmelweis, Joseph Lister and John Snow were a great contribution to the acceptance of germ theory. However, the laboratory works of Louis Pasteur in the 1860s and Robert Koch in the following decades, provided the scientific proof for germ theory. Their works opened the door to research related to the identification of disease-causing germs and potential life-saving treatments.

15.1.1 The work of Louis Pasteur and Semmelweiss

The work of Eduard Jenner and Ignas Semmelweiss showed that infectious diseases maybe caused by an infectious agent or germ. This was accepted as the germ theory of disease for a very long time. The work of Robert Koch and Louis Pasteur led to a wide acceptance of the germ theory

Pasteur made a number of important steps forward.  He indicated that fermentation is the result of the action of microorganisms (Yeast) on sugar. Huge number of people at the time believed that living things could arise spontaneously from non-living things and this theory is known as spontaneous generation. To reject this theory, he showed that if broth is boiled in a sealed container, it would stay clear, but once he added material which had been exposed to the air, microorganisms grew in the broth. Finally, he designed a series of experiments using swan necked flasks which showed once and for all that any microorganisms which appear in boiled broth come from the air not arise spontaneously from nonliving organism such as broth.

In 1845-1846, Pasteur found a way of avoiding the disease of silkworms by observing the infected eggs under microscope and thereby saving the silk industry. This was the first clear evidence of microorganisms causing disease. Pasteur even developed vaccines against a number of these diseases.

15.1.2 Summary of the contributions of Louis Pasteur in microbiology and medicine

The contribution of Louis Pasteur in microbiology and medicine also include:

–– The fight against spontaneous generation theory
–– The technique of sterile culturing of microorganisms
–– The technique of fermentation and conservation of drinks. This technique is known as Pasteurization.
–– The technique of antiseptic surgery to prevent contamination of wounds during the surgical operations in hospitals.
Louis Pasteur developed the germ theory of disease which postulates that all contagious and infectious diseases must be caused by pathogenic microorganisms.

15.1.3 The Germ Theory and Koch’s Postulates

Diseases can be spread by air, water, food, and human as well as animal vectors. In, an english physician called John Snow (1854) and a German microbiologist called Robert Koch (1884) found a relationship between polluted water and disease. Robert Koch, has isolated the bacterium Vibrio cholera, the cause of cholera from Elbe River water to provide the relationship.

Koch went on to formulate an established set of procedures to isolate and identify the causative agent of a particular microbial disease. The following four steps, which are still used today, are known as Koch’s Postulates:

Postulate 1: A specific organism must always be observed in association with the disease.
Postulate 2: The organism must be isolated from an infected host and grown in pure culture in the laboratory.
Postulate 3: When the organism from the pure culture is inoculated into a susceptible host organism, it must cause the disease.
Postulate 4: The infectious organism must be re-isolated from the diseased organism and grown in pure culture.

Self-assessment 15.1
1. What are the Koch’s Postulates?
2. Explain the theory of spontaneous generation.

15.2 Classification of diseases
Activity 15.2
Use the Knowledge gained to answer to the following questions:
1. Propose two infectious diseases and for each disease, give:
a. their causal agents
b. causal agents’ type
c. their symptoms
d. their methods of prevention
e. Treatment
2. Does being healthy means just the absence of the disease? Explain.
The following are the meaning of disease, signs and symptoms:
–– Disease is the disruption of normal body function.
–– Signs are indications of a disease that can be observed by examining the patient.
–– Symptoms are indications of disease perceive only by the patient.
The normal functioning of the body is disturbed, when the body is infected. Many types of diseases are broadly divided into two categories: Infectious diseases and non-infectious diseases

15.2.1 Infectious diseases
Infectious diseases are caused by microorganisms known as pathogens which may include viruses, bacteria, fungi and protozoa. Those diseases are called communicable diseases. as they can be transmitted from one person to another. They include cholera, malaria, typhoid, HIV and AIDS…Malaria is one of the most dangerous infectious diseases, endemic in Latin America, Africa and South-East Asia. Some infectious diseases can also be from animals to humans.

The following are some technical terms used when discussing about infectious disease.

–– Aetiology: The study of the cause of disease.
–– Epidemiology: The study of all the factors that contribute to the appearance of a particular disease
–– Causative agent: The organism which causes the disease
–– Vector: An organism which carries the causative agent of the disease from one person to another or from infected animal to human.
–– Incubation period: The period of time between the original infection and the appearance of signs and symptoms.
–– Infective period: The time during which a person is capable of passing the
disease on to another person.
–– Carrier: The person who has been infected but develop no signs and symptom, the carrier can pass the disease on to another person
–– Prevention: Measures taken to prevent diseases.
–– Treatment: Measures taken to cure diseases Antibody: Is a protein produced by the body’s immune system when it detects harmful substances called antigen.
–– Antigen: Is any substance that causes your immune system to produce antibodies against it.
–– Host: A host can be anything living organism ion which pathogens can survive
–– Hygiene: Practices that help to maintain health and prevent the spread of diseases
–– Immunity: Is the ability of the body to resist to infections.

a. Important advances in the work against infectious disease

Table 15.1: The chronology of some of the most important advances in the work against infectious disease

b. Some groups of communicable diseases
– Bacterial diseases: these are diseases caused by bacteria. They include cholera, typhoid, tetanus, tuberculosis, etc.
– Viral diseases: these are diseases caused by viruses. They include AIDS, polio, measles, Ebola, etc.
– Protozoan diseases: these are diseases caused by protozoa. They include malaria, sleeping sickness, trichomoniasis, etc.
– Fungal diseases: these are diseases caused by fungi. They include candidiasis, athlete’s foot, ring worms, etc.
– Worm diseases: these are diseases caused by worms. They include elephantiasis, bilharzias, etc.
– Sexually transmitted diseases: these are diseases transmitted through sexual contact. They include HIV-AIDS, syphilis, gonorrhea, etc.

c. Transmission of infectious diseases

Pathogens can spread when you have direct contact with an infected person. For example, if you have contact with the person’s blood, body fl uids or open wounds. Pathogens can also be spread through contaminated food, water or air. Infected animals can spread pathogens to people.

The following conditions lead to the spread of an infectious disease:

–– A pathogen which causes the disease.
–– A source which is an infected organism.

Mode of transmission a pathogen must be able to enter the body of the new host to cause an infection. Infectious diseases follow a pattern of development from the time of infection. The pattern of development has five distinct periods, as described in table below:

d. Epidemiology

The study of patterns of disease and of the various factors that affect the spread of disease is called epidemiology.

Epidemiologists try to discover the factors that cause a disease and develop methods to prevent its spread. The main clue they use come from data about the number of people in a particular area affected by specific diseases, and the number of death The data are commonly expressed as incidence or morbidity and mortality rates.

The incidence rate is the number of new cases of disease in a given population occurring during a specific period (a week, month or a year). It is calculated as:

Example: among 900 students at a given school, 50 suffer from malaria and 4 deaths were recorded. Calculate the morbidity or the incidence of malaria at that school.

To find how many cases of a disease are new, this calculation requires information about the prevalence rate. This is the total number of individuals infected in a population at any one time.

The mortality rate of a disease may be estimated for a whole population irrespective of whether they have the disease or not.

It may be calculated by using only those people who have disease.

To make fair comparisons between different populations with the same disease, epidemiological information is usually adjusted. For example, the mortality rate among those with a particular disease is usually expressed as a percentage or ratio per year. Hence, if in one year 7500 people in a given area die as a result of AIDS and the total number of population infected was 30 000, the mortality rate would be 25% for the rate.

Epidemiological studies are used to identify whether a disease is endemic, epidemic, or pandemic:

–– Endemic disease is a disease that is always present in a people e.g. malaria in
tropical Africa.
–– Epidemic disease is a disease that spreads rapidly, suddenly, and unexpectedly
to affect many people. e.g. cholera in
refugees’ camp. Pandemic disease is a disease that affects people over very
large area, such as a continent or even the whole world e.g. AIDS and TB are
pandemic at present

15.2.2 Non-infectious diseases

These  diseases are also called non-communicable diseases. They cannot be transmitted from one person to other examples: albinism, kwashiorkor, cancer, diabetes, etc

Lesson self-assessment 15.2
1. Answer by true or false
a. Epidemic disease is a disease that is always present in population.
b. The diseases that transmitted among people by pathogens are called transmissible diseases
c. The study of patterns of disease and of the various factors that affect the spread of disease is called epidemiology.
d. Cholera is infectious disease
e. Malaria is non-infectious disease whose vector is mosquito.
2. Distinguish between morbidity and mortality

15.3 Common infectious diseases

Activity 15.3

Choose in the following list the infectious diseases and explain why:

Cholera, typhoid, Alcoholism, tetanus, tuberculosis, AIDS, Haemophilia, polio, measles, Ebola, malaria, Anorexia, obesity, sleeping sickness, trichomoniasis, sickle-cell anaemia, candidiasis, athlete’s foot, ring worms, elephantiasis, bilharzias, syphilis, gonorrhea, cystic fibrosis, Arthritis, Anorexia, schizophrenia.

15.3.1 Measles

Measles is a contagious acute viral disease with symptoms that include a bright red rash of small spots that spread to cover the whole body. Small white spots, known as Koplik’s spots, appear in the mouth on the inside of the cheeks a few days before the rash appears and can be used in diagnosis.

Failure to eradicate measles

–– Incubation period is short hence it is difficult to identify and isolate before they become infectious
–– It is transmitted through a carrier mother to healthy children hence it is hard to eradicate.
–– It targets young children who like playing together. This makes ease the spread of the disease.

15.2.3 Typhoid

a. Causal agent of typhoid

Typhoid is waterborne disease caused by Salmonella typhus, a Gramnegative bacterium. The bacteria are derived from the feces of a patient. It has high infectivity as low dosage of organisms is only needed for typhoid to spread. Common sources of typhoid infection are contaminated water, milk and food.

b. Development of typhoid illness

The incubation period is of ten days. After this time, the germs enter the bloodstream and the patient develops the following symptoms: Headache, Muscular pains, Fever reaching its peak after about 1 week, faint rash may appear, diarrhea in the second week, mental confusion, etc. The third week shows the peak of the illness and the patient may die if not treated.

c. Treatment and prevention of typhoid

The disease had a 20% fatality rate before the use of antibiotics. Chloramphenicol and Ampicillin are effective and reduce fatality rate from 1 up 5%. Today, Ciprofloxin is used as another antibiotic. The two most important preventive measures are; proper treatment of sewage and disinfection of water supplies. Hygienic measures in the food trade and at home, and control of flies, which can transfer fecal material to food. Vaccine is formed by a polysaccharide from the capsule of the bacteria.

15.2.4 Cholera.

Cholera is a good example of a waterborne disease. It is endemic in parts of Asia, particularly India. The organism which causes cholera is a comma shaped motile bacterium called Vibrio cholerae.

a. Transmission and symptoms of cholera

The main source of infection is water contaminated by feces with Vibrios. It is estimated that only about one infected person in 50 develops the disease, the rest being carriers. Drinking contaminated water, or washing food or utensils in it, is the most common means of transmission. Direct contamination of food with feces as a result of poor hygiene is also possible, house flies being the main vector in this last case.

b. Signs and symptoms of cholera

Vibrio cholerae multiply in the intestine, releasing a powerful toxin which results in violent inflammation of the intestine and production of the watery diarrhea.

The main sign of the disease is severe diarrhea due to irritation of the bowel by toxins from the Vibrios. The liquid of the feces is so profuse and cloudy like “rice water”.

Abdominal pain and vomiting are also common. Dehydration is rapid and quickly results in death unless rehydration treatment is given. Fever is absent; in fact, the skin feels deathly cold and often damp.

c. Treatment of cholera

The primary cause of death from cholera is dehydration i.e. loss of water with its minerals salts. For that, it is obligatory to rehydrate with oral serum which contain mineral salts and sugar.

The loss fluid may be replaced by administration of a drip food into a vein. Various antibiotics, such as tetracycline’s and chloramphenicol, are used to treat cholera. Chloramphenicol is effective against tetracycline-resistant Vibrios.

d. Prevention of cholera

–– Use clean drinking water,
–– Proper treatment of sewage and sanitation
–– High standards of public and personal hygiene, particularly in relation to food
(such as washing hands after defecation)
–– Health education
–– Vaccination is recommended for people visiting areas where cholera is
endemic and for those living in such areas. But the vaccine lasts few months.
–– Isolation of patients and hygienic disposal of feces and vomit from patients.

e. Failure to eradicate cholera
–– Vaccination is not very effective
–– It is a waterborne disease i.e. transmitted through contaminated water
–– Poor sanitation condition in camps.

15.2.5 Tuberculosis (T.B)