Why is the species-diversity of deadly parasites greatest in the tropics?

Question

There are so many parasites living in tropical regions of Africa, South America, or Asia, but very few in Europe or North America. Is this due to climate, or are there other reasons? Many of the tropical diseases and parasites are transmitted by insects, such as flies and mosquitoes. Well there are flies and mosquitoes in Europe as well. There might be malaria-transmitting mosquitoes in the very south of Europe, and there is encephalitis transmitted by ticks. But that's it. Why don't the hundreds of different parasitic species from Africa spread to Europe ?

Answer 1

Each of the diseases usually associates with a particular host, and those hosts have not adapted to cold environments. So there isn't chagas disease in say, Alaska, because even if one of the hosts were to hitchhike onto someone's boot, as soon as it got off the plane in Fairbanks, it would freeze to death. The same goes for mosquitoes that carry trypanosomosis, and dengue, etc. The winter frost kills the disease vectors.

That being said, new associations could always spread, cold-hardy strains of mosquitoes/other hosts could evolve, and the climate could warm leading to expanded ranges of the vectors.