Malaria is caused by a single-celled organism with a complex life cycle typical of many parasites. It exists in different forms in different stages in its life and the form that lives in a mosquito is quite different from the form that lives in a human. If you think about how it is transmitted you will see that this might well be the case: it infects the mosquito by being eaten; but it infects a human by being transferred from mosquito saliva into human blood.

Any parasitic life-form will be unsuccessful if it causes its hosts to drop dead immediately. It is not in the interests of the plasmodium (which causes malaria) to make the mosquito so ill that it can't bite a human. Many examples of multi-host parasites might be of interest to you - you will quickly see parallels between them. Often the parasitic creature can seem almost sinister in its intentions, but, of course, it doesn't have intentions, just a need to live and to reproduce.

As far as I know, rats and other rodents do get ill with bubonic plague. The fleas that live on them leap off to find other hosts if the rodent dies. It is the change of host by the flea that transmits the bacteria to a new victim, be it another rat or a human.

Good luck with your studies - I hope they aren't all so gruesome!

Pete

This is because the mosquito and malaria > rat and bubonic plague have a symbiotic relationship. This means: two dissimilar organisms living together in a mutually beneficial relationship. They each help each other survive. The malaria uses the mosquito as a primary host to carry it to a secondary host and is layed inside it. It does not harm the mosquito (or in the other case the rat) it just uses it as a neutral host.

Congratulations to the children of the Christopher Hatton School in London, UK, for asking such a penetrating question.

Rats can get quite sick from plague fleas and some will die, but usually not too quickly. Plague-carrying rats are at their most dangerous when they are about to die, because their fleas leave them as soon as they are dead to find new hosts.

The malaria parasite Plasmodium does not usually kill its host mosquitoes, though it may take a high enough toll that it is better for the mosquitoes not to get infected.

If we could breed mosquitoes that were resistant to the parasite we might find that they outcompete ordinary mosquitoes, and this might ultimately help get rid of malaria. This kind of strategy would not work with yellow fever, as the mosquitoes that carry the virus responsible for the illness in humans hardly seem to be affected.

Jon Richfield, Somerset West, South Africa

Plasmodium, the parasite that causes malaria in humans, infects mosquitoes. The mosquitoes then transmit it to people when feeding on their blood. As for the plague microbe, Yersinia, it blocks the gut of the flea that transmits it. As a result, when an infected flea feeds on the blood of a human or rat, it will regurgitate some blood containing the microbe and so spread the germ to a new host.

To address the question directly, the important thing to note is that being infected with a microbe or other parasite does not necessarily cause disease, because it is often in the interest of the microbe to cause no harm to its hosts.

However, the mosquitoes that transmit Plasmodium are affected by it, as the parasite grows in their salivary glands. Such infection can reduce the ability of the salivary glands to function and thus the viability of the mosquitoes.

A related parasite called Theileria, transmitted between cattle by ticks, can damage the gut and salivary glands of the ticks, and can even kill them in the laboratory. Epidemiologists make a point of studying the extent of such effects under natural conditions.

Alan R. Walker, Edinburgh, UK