This broad medical field is concerned with the prevention, diagnosis and treatment of diseases that afflict people who live in or visit tropical areas of the world. The tropics are a breeding ground for truly nasty pathogens; tomorrow's global epidemic may be just a local swampland plague today, so the field is very important to human medicine as a whole.
The diseases studied and treated in tropical medicine include:
The field also concerns itself with international health policies, health economics, and epidemiology.
From the science dictionary at http://biotech.icmb.utexas.edu/