Just one bite, by one infected mosquito is enough, and despite everything mentioned in previous articles, you could be unlucky. It takes about a week to go from the bite to the disease if nothing intervenes. Conversely, you could employ some ineffective scheme that appeared to work, simply because you were lucky. Or even do nothing and be lucky. It is a matter of chance, and again, single histories have little bearing on assessments of the probabilities.

What is clear however, is that the longer you spend in a malarial area, the more likely it will be that you cop an unlucky bite. Even in the worst areas, no more than a few percent of the mosquitoes are infective, but if you are accumulating hundreds of bites, that is small comfort.

Your second line of defence involves internal chemicals. Certain drugs are taken throughout the time there is a chance that an infective mosquito might deliver parasites to you. This prophylaxis however does not kill the parasite, it merely prevents it from breeding. For this reason, the drugs must be continued for four weeks past your departure from a malarial zone so that they die of old age without issue.

To establish a suitable level of the drug in your system ready for any early arrivals, the drug must be started perhaps two weeks before you enter a malarial zone (chloroquine), or four weeks for mefloquine in part to allow side effects to be assessed before you leave. If your travels take you out of a malarial zone for a while, then you return, the gap would have to approach two months before it would be worth stopping the drugs.

During the time that you are taking the drugs, you may well be host to the parasite, perhaps from a bite you haven't noticed. While the parasite numbers are small, you will have no symptoms, nor will a blood test be certain to detect any parasites. The drugs do not suppress the symptoms, they suppress the parasite that causes the symptoms. Thus, finish the final four weeks, even though you're comfortable at home. If, after your return you develop any fever, you must warn your doctor that you've been to malarial areas as otherwise malaria is likely not to be considered and you can become dangerously ill. There is a slim possibility that malaria will develop months after departing a malarial zone and completing your post-exposure course, as one stage in the parasite's complex life cycle takes place in your liver, and some parasites can remain dormant there rather than breed at once.

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