When climate patterns change, disease patterns also change, especially those diseases that rely on an animal vector. These include malaria, sleeping sickness, leishmaniasis, elephantiasis and many other diseases with vectors that rely on humans to complete a part of their life cycle. Research has shown that even small changes in temperature can cause insect populations to increase, coupled with the fact that the parasites inside the insects can mature more quickly when temperatures are higher. A rate of one means that one person will infect one other person, which will keep a disease ticking over, but will not really cause any major problems on a population level. Mumps and diphtheria are more infectious, and the rate is about eight.
Epidemics can quickly get going when the rates of infection are high.
The rate of malaria is estimated to be about 90 - which is why an increase in insect populations could have such a devastating effect. In America and most of northern Europe, most of these serious insect-vector diseases have been eradicated, and health authorities have moved on to worrying about other things. In America in 1991, there were 1,000 cases of malaria, most of them contracted abroad, but recently, there have been reports of people getting malaria who were in contact with people who had been abroad, but who had not been abroad themselves. Results so far have not been as predicted, mainly due to the fact that incidence of disease and the tsetse, and climate, vary from region to region. In Zimbabwe, there were more tsetse in the hottest months of the year.