Infectious Diseases, Immunity and Vaccination
Senior 2 · Biology
Infectious Diseases, Immunity and Vaccination
Lesson 1 of 5
20 min
Think of the last time someone at home had a fever, diarrhoea or a cough that would not go away. Illnesses like these are infectious diseases: they are caused by a living organism, or a virus, that has entered the body and can pass from one person to another. Illnesses such as sickle-cell anaemia or type 2 diabetes cannot be caught from anyone; they are non-infectious.
Because infectious diseases travel, they can also be stopped, if you know exactly how they travel. Health workers in Rwanda do this every day: they find where an infection comes from, work out its route, and block it. In this lesson you will learn to do the same thinking.
A pathogen is any microorganism or parasite that causes disease. The organism it lives in and harms is its host. Pathogens belong to five main groups.
| Group | What it is | Examples of diseases |
|---|---|---|
| Viruses | Tiny particles made of genetic material inside a protein coat. They can reproduce only inside a living host cell. | influenza, measles, polio, HIV, COVID-19 |
| Bacteria | Single-celled organisms with no nucleus | cholera, typhoid, tuberculosis (TB), gonorrhoea |
| Protists (protozoa) | Single-celled organisms that do have a nucleus | malaria (caused by Plasmodium), amoebic dysentery |
| Fungi | Organisms that feed on living or dead tissue; many grow as fine threads | ringworm, athlete's foot, oral thrush |
| Parasitic worms | Many-celled animals that live in or on the body | roundworm, hookworm, bilharzia (schistosomiasis) |
A pathogen must survive between one host and the next. The place where it normally lives is its reservoir. A reservoir can be a person, an animal (the rabies virus lives in dogs), soil (the tetanus bacterium survives there for years as a tough spore) or water (cholera bacteria in water polluted with faeces).
A carrier is a person who has a pathogen and can pass it on, but who may have no symptoms at all. Carriers are dangerous because nobody suspects them. HIV and hepatitis B, for example, are often passed on by people who look and feel healthy. The time between catching a pathogen and the first symptoms is the incubation period.
After heavy rain, a pit latrine in a village overflows into the stream that families use for drinking water. Three days later, several families have sudden, very watery diarrhoea. A health worker tests samples and finds the bacterium Vibrio cholerae. Work through the chain one link at a time.
Notice the method. You do not need a medicine to stop an outbreak. You need to cut just one link in the chain between the reservoir and the next host.
Tip
Pathogen → route → barrier. In any exam question about disease, answer three things in this order: what kind of pathogen it is, how it travels, and which barrier blocks that route.
Try each one on paper before you read the answer.
Name the group of pathogen that causes each disease: (a) ringworm (b) measles (c) bilharzia (d) typhoid.
Answer: (a) a fungus; (b) a virus; (c) a parasitic worm; (d) a bacterium (Salmonella Typhi).
Explain why the female Anopheles mosquito is called a biological vector but the housefly is called a mechanical vector.
Answer: The malaria parasite lives and develops inside the mosquito's body before the mosquito injects it with a bite, so the mosquito is itself infected: a biological vector. The housefly is not infected; it only carries bacteria on its feet and mouthparts from faeces to food: a mechanical vector.
A student with untreated lung TB coughs in a crowded classroom with the windows shut. Name the route of transmission and give two ways to reduce the risk to classmates.
Answer: The route is airborne. To reduce the risk: open the windows and doors for ventilation; the student should be tested, start treatment and stay at home until a health worker says they are no longer infectious; everyone should cover coughs.
Explain why a carrier with no symptoms can be more dangerous to a community than a person who is clearly ill.
Answer: A person who is clearly ill usually rests at home and gets treated, so others are careful around them. A carrier feels well, goes to school, work and the market as usual, and nobody takes precautions, so the carrier can pass the pathogen to many people without anyone knowing.
Remember
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