Contents: 9 sections
Syllabus points
- State the name and type of pathogen that causes cholera, malaria, tuberculosis, HIV/AIDS.
- Explain how each is transmitted.
- Discuss the biological, social and economic factors in their prevention and control.
What an infectious disease is
An infectious disease is caused by a pathogen, an organism that lives in or on a host and causes harm, and it can be transmitted from one host to another.
That distinguishes it from a non-infectious disease such as scurvy or a genetic condition, which cannot pass between people.
The four diseases
Learn each as a set of four facts: pathogen, type, where it lives in the body, and how it is transmitted. Questions almost always test one of those four.
| Disease | Pathogen | Type | Transmission |
|---|---|---|---|
| Cholera | Vibrio cholerae | bacterium | water and food contaminated with faeces |
| Malaria | Plasmodium (four species) | protoctist | bite of an infected female Anopheles mosquito, also unscreened blood and across the placenta |
| Tuberculosis | Mycobacterium tuberculosis, also M. bovis | bacterium | airborne droplets, and unpasteurised milk from infected cattle |
| HIV/AIDS | human immunodeficiency virus | virus | unprotected sex, infected blood and needles, breast milk, across the placenta |
Two of these need care. Malaria is not caused by the mosquito; the mosquito is the vector that carries the pathogen. And HIV is the virus while AIDS is the condition it eventually produces, so they are not interchangeable words.
Cholera
Vibrio cholerae is swallowed in contaminated water or food. Most bacteria are killed by stomach acid, which is why a large dose is usually needed for infection. Survivors reach the small intestine, pass through the mucus lining and attach to the epithelium.
There they release a toxin. The toxin causes chloride ions to be actively pumped out of the epithelial cells into the gut lumen. The rise in ion concentration lowers the water potential of the gut contents, so water leaves the cells and the surrounding tissues by osmosis.
The result is severe watery diarrhoea, which is why the danger is dehydration and loss of ions rather than the bacterium itself. Untreated, death can follow within a day.
Treatment is oral rehydration therapy: a solution of water, salts and glucose. Glucose is included because it is absorbed by co-transport along with sodium ions, which drags the ions and water back across the gut wall.
Control depends on clean water supply and sewage treatment, which is why cholera is a disease of poverty, disaster and displacement rather than of any particular climate.
Malaria
Plasmodium has a life cycle in two hosts. The female Anopheles mosquito is the vector, and she needs a blood meal to develop her eggs.
When she bites an infected person she takes up Plasmodium gametes. The parasite reproduces inside her, and when she next bites, infective stages pass in her saliva into the new host. In the human the parasite multiplies in the liver and then inside red blood cells, which burst and release more parasites, producing the recurring fever.
Control attacks the cycle at several points:
- Reducing the vector: draining or spraying standing water where larvae develop, releasing fish that eat larvae, and spraying insecticide.
- Avoiding bites: insecticide-treated bed nets, screens, and repellent.
- Drugs: prophylactics taken by travellers, and treatment for those infected.
The difficulties are biological and economic together. Plasmodium has developed resistance to drugs and mosquitoes have developed resistance to insecticides. Nets have to be replaced and re-treated, which requires money and organisation. There is no long-lasting vaccine in wide use, partly because the parasite changes its surface antigens.
Tuberculosis
Mycobacterium tuberculosis is spread in droplets coughed or sneezed by an infected person and breathed in by another. Crowded, poorly ventilated housing therefore raises transmission sharply.
The bacterium is engulfed by phagocytes but survives inside them, because its thick waxy wall resists the enzymes. It can remain dormant for years, walled off in the lungs, and become active later if the immune system weakens. That is why HIV infection so often leads to active TB.
Control involves:
- the BCG vaccine
- contact tracing to find and test people who have been exposed
- treatment with a combination of antibiotics for six to nine months
- improving housing and nutrition
The long course of treatment is the practical problem. Patients feel better after a few weeks and stop taking the drugs, and the bacteria that survive are the more resistant ones. That is precisely how multi-drug resistant TB arises. DOTS, directly observed treatment short course, addresses it by having a health worker watch each dose being taken.
HIV and AIDS
HIV infects and destroys T helper lymphocytes, the cells that coordinate the immune response. As their number falls, the immune system loses its ability to respond to anything.
AIDS is diagnosed when the T helper cell count has fallen far enough that opportunistic infections appear: infections that a healthy immune system would deal with easily, such as certain pneumonias, and tuberculosis.
People die of those opportunistic infections rather than of HIV directly.
Control is difficult for reasons that are largely not biological:
- there is no vaccine, because the virus mutates its surface proteins rapidly
- antiretroviral drugs control the infection but do not cure it, and must be taken for life
- there is a long asymptomatic period, so people transmit the virus without knowing they carry it
- social stigma discourages testing, and honest contact tracing is hard when the routes of transmission are private
Measures that work are education, free condoms, needle exchange, screening blood, and testing and treating pregnant women to prevent transmission to the baby.
Why these diseases persist
The pattern across all four is that the biology is understood and the obstacles are social and economic.
- Poverty: no clean water for cholera, crowded housing for TB, no money for nets or drugs.
- Health infrastructure: too few clinics, too few trained staff, no reliable supply of drugs, no refrigeration for vaccines.
- Movement of people: war, disaster and migration spread disease and break up treatment programmes.
- Education: if transmission is not understood, prevention is not practised.
- Resistance: to antibiotics, to antimalarial drugs, and to insecticides, all made worse by incomplete treatment.
- Cultural factors: stigma around HIV, and resistance to some control measures.
A question asking why a disease has not been eradicated wants factors from more than one of those categories, not just the biology.
Common mistakes
- Saying malaria is caused by a mosquito. It is caused by Plasmodium; the mosquito is the vector.
- Saying HIV and AIDS are the same thing. HIV is the virus; AIDS is the condition that follows.
- Saying cholera kills through the bacterium spreading round the body. It stays in the gut; the killer is dehydration from the toxin.
- Saying TB is treated with a single antibiotic for a week. It needs a combination, for months.
- Naming only biological control measures when the question asks about prevention. The social and economic factors carry as many marks.