When I did a plasma donation the other day, the blood-bank people put a note on my file so that the plasma would be fractionated & treated as necessary to prevent any recipients from getting malaria. (Not that I"ve got malaria!) This was because I'd just got back from Vanuatu, and malaria is relatively common on the outlying islands there, so you're advised to take anti-malarial drugs as a prophylactic measure (a preventative) if you're travelling in the region. (We didn't, because we were going only to the main island, & the resorts tend to have a fairly vigorous mosquito-control program. We never saw a mozzie while we were there.)
Now there's news (Enserink, 2008) that the malarial parasite, Plasmodium, is showing signs of developing resistance to the most commonly-used anti-malarial therapies - an example of evolution in action. Scientists working on the Thai-Cambodian border have found that these drugs, called artemesinins, are not clearing the parasites from patients' blood as quickly as they used to: a sign that the parasite is becoming tolerant of the drugs.
This isn't a new development, in the sense that artemesinins are only the latest anti-malarial drugs in our armoury. But evolution in Plasmodium has previously rendered all our earlier drugs useless (and again, this resistance evolved in the Thailand/Cambodia region before spreading round the world).
And why is this? One posible explanation is that people in that region either aren't getting the right drugs at the right doses, or they're getting fake drugs that aren't as effective as the real thing. In either case, this would mean that in these patients Plasmodium is being subjected to strong selection pressures, with the outcome an increase in resistance to the drugs. From there, it seems likely that travellers & migrants have taken their resistant parasites with them to other parts of South-East Asia, & thence to other continents. Not so good for us, but a real advantage for Plasmodium.
M. Enserink (2008) Signs of drug resistance rattle experts, trigger bold plan. Science 322: 1776