The first great world killer is mosquitos who pass around malaria, yellow fever, and a variety of other deadly diseases - although in North America, Deer are involved in more deadly accidents through car wrecks, than people dying of malaria.
The second big killer is not particularly well known:
Tommy Leung, Parasite of the Day (blog), 17 February 2013 (no ht)
Blood flukes from the Schistosoma genus is found in over 77 countries, infecting at least 230 million people, and second only to malaria as the most socioeconomically crippling parasitic disease in the world. But the majority of flukes from the the Schistosomatidae family do not infect humans; they parasitise other species of mammals, as well as birds. There are about 100 known species of schistosome flukes around the world. Understandably, those species from the genus Schistosoma are the most extensively studied due to their public health importance. However, there are many other blood flukes for which very little is known on even the most basic aspect of their ecology.
The post above is about elephant blood flukes.
Blood flukes are found in warm areas. The human version, like malaria, it is a two animal disease, with the babies (larva) living in water snails, and the adults living in the abdomen.
They aren't found in North America. I imagine they don't do well in cold climates in general.
But unless you are one of the religious Fox News worshipers I know ("I just don't buy into global warming"), you may have had some thoughts about the weather getting a bit warmer lately.