The United States' latest brush with Ebola shows that, more than ever, diseases are global travelers.

A physician died at a Nebraska hospital on Monday after contracting the disease in Sierra Leone, where he was working. A New York physician came down with Ebola last month after working in West Africa, sparking quarantines for returning health workers in some states. Earlier in October, a man died in Dallas after becoming infected in Liberia.

But Ebola isn't the tropical disease that's most likely to cause health problems in the United States—not by a long shot. A handful of other tropical diseases pose much more realistic threats. And they're spread by insects, which can't be quarantined.

Until now, most of the cases in