Why is the species-diversity of deadly parasites greatest in the tropics?
The diversity of parasites shows a gradient with increasing diversity from the poles to the equator. Several reasons have been brought forth to explain the latitude-dependency of parasite diversity:


An increased diversity overall around the equator; species diversity in general is greater in the rain forests and hence more hosts are available and thus more parasitic species can develop that target specific hosts;
Larger amounts of precipitation and higher temperatures around the equator may favor the development and transmission of parasites (Nunn et al., 2005);
Increased available energy overall around the equator (Guernier et al., 2004).


References
- Guernier et al., PLOSone (2004): 0020141
- Nunn et al., Diversity and Distrib (2005); 11: 249–56