Diseases Caused by Pathogens

Before the diseases themselves, it helps to fix the five great families of pathogen in mind, because almost every question is really asking which family a disease belongs to. The smallest are the viruses, which are not even fully living on their own and can only reproduce inside the living cell of a host; next come the bacteria, single-celled organisms that can live and multiply on their own; then the protozoa, single-celled animals mostly spread by insect vectors; the fungi, plant-like organisms with a cell wall of chitin; and finally the worms or helminths, large multi-celled parasites that usually live in the intestine.

• Bacteria are single-celled organisms that reproduce on their own, inside or outside the body.
• Fungi are plant-like with a cell wall of chitin and reproduce by spores; protozoa are microscopic and mostly water-borne.
• A chemical poison such as arsenic is NOT a pathogen — a pathogen must be an agent of infection.
• The single best defence is good hygiene, clean water and timely vaccination against every family on this list.

Viruses are the smallest pathogens, and because they hijack the body's own cells to multiply, viral diseases generally cannot be cured by antibiotics and are fought instead with vaccines, rest and fluids. Many of the most heavily examined viral diseases are mosquito-borne, while others spread by droplets, contact or contaminated water.

• Zika, dengue and yellow fever are all mosquito-borne viral diseases carried by Aedes.
• HIV spreads by sexual contact, infected blood and from mother to child.
• Nipah virus spreads from animals — fruit bats and pigs — to humans.
• Polio spreads through poor sanitation and contaminated water/food.
• Dengue is a VIRUS spread by Aedes, but malaria is a PROTOZOAN spread by Anopheles — never confuse the two.
• AIDS is the disease while HIV is the virus that causes it, attacking the immune system.
• Viral diseases are prevented by vaccination, hygiene and avoiding contact.

Bacteria are single-celled organisms that, unlike viruses, can live and reproduce on their own and can be killed by antibiotics, which is the standard exam cue that a disease is bacterial. They spread chiefly by contaminated food and water (gut diseases such as cholera and typhoid) or by droplets and close contact (lung diseases such as tuberculosis and pneumonia).

• Cholera is caused by Vibrio cholerae, spread by contaminated water and poor sanitation.
• Typhoid is caused by Salmonella typhi, spread by contaminated food and water.
• Leprosy is caused by Mycobacterium leprae and attacks skin and nerves.
• Bacteria are classified by shape — cocci (spherical), bacilli (rod), spirilla (spiral).
• Anthrax is a serious, sometimes fatal disease caught from infected animals/meat.
• Bacterial diseases can be treated with antibiotics, plus clean water and vaccination for prevention.

Protozoa are single-celled animals, larger than bacteria, and the diseases they cause are most often spread by an insect vector — a mosquito, a sandfly or a fly. The single most important fact in the whole topic lives here.

• The Plasmodium parasite that causes malaria is neither a virus nor a bacterium.
• Kala-azar (Leishmaniasis) is spread by the sandfly.
• Sleeping sickness is spread by the tsetse fly and affects the nervous system.
• Amoebic dysentery is caused by Entamoeba histolytica.
• Many protozoan diseases are spread by insect vectors — mosquitoes, sandflies and flies.

Fungi are plant-like organisms with a cell wall made of chitin that reproduce by spores, and the diseases they cause are mostly infections of the skin, hair and nails, treated with antifungal medicines.

• Athlete's foot is common in people who wear tight shoes for a long time.
• Fungi reproduce by spores and have a cell wall of chitin.
• Some fungi are helpful — yeast is used in bread making — but others cause disease.
• Avoid sharing towels and shoes to prevent fungal infections; fungal infections are treated with antifungal medicines.
• Rust of wheat reduces crop yield and affects food production.

The largest pathogens are the worms, or helminths — multi-celled parasites that, unlike all the microbes above, can often be seen with the naked eye and usually live in the human intestine, where they are removed with deworming medicines.

• Filariasis (elephantiasis) is spread by the Culex mosquito and causes gross swelling of the limbs.
• Worms are multicellular parasites that mostly live in the intestines of hosts.
• Worm infections are treated by deworming medicines.

Examiners like to test the broad principle that the treatment must match the family of pathogen, since the wrong medicine simply does not work.

• Fungal infections are treated with antifungal medicines; worm infections with deworming medicines.
• Early detection and proper treatment can save lives.
• Good hygiene, clean water, safe food, vaccination and vector control prevent most diseases — prevention is better than cure.