Types of microorganisms
Bacteria are single-celled organisms without a nucleus; some cause disease, but many are harmless or even beneficial. Viruses are much smaller than bacteria and can only reproduce inside a living host cell, hijacking its machinery. Fungi include moulds and yeasts; some cause diseases like athlete's foot or ringworm. Protozoa are single-celled organisms with a nucleus; some, like the Plasmodium parasite, cause serious diseases.
Disease transmission
Diseases spread through direct contact (touching an infected person or object), airborne droplets (coughing, sneezing), contaminated water or food, or via a vector -- an organism that carries the pathogen from one host to another.
Example 1: Explain how malaria is transmitted, and identify the vector involved.
Malaria is caused by the Plasmodium parasite and transmitted by the bite of an infected female Anopheles mosquito, which acts as the vector -- the mosquito picks up the parasite from one infected person's blood and transmits it to the next person it bites.
Immunity and vaccination
The body's immune system can develop immunity to a pathogen after exposure, by producing antibodies that recognize and help destroy it, and by "remembering" the pathogen for a faster response if it appears again. A vaccine triggers this same memory response using a weakened, inactive, or partial form of the pathogen that cannot cause the actual disease, giving protection without the risk of getting sick first.