Tonight, I attended one of the meetings of the University of Pittsburgh School of Medicine's "Bug Club". The members of this club are largely 1st and 2nd year Pitt med students who have a general interest in infectious diseases, especially of the tropical flavor.
The focus of the meeting tonight was on schistosomiasis, an awesome disease caused by a parasitic worm that jumps between humans and snails.
One of the most intriguing aspects of this disease is how the worm finds its human host. Schistosomes emerge from snails in freshwater and, after finding a human, penetrate human skin to begin the process of infection. To detect that a human is in their environ, the organism "smells" human skin, i.e. the molecules on the skin of a human (e.g. linoleic acid). This attraction is so great that it sometimes prompts them to reverse the direction in which they are swimming.
Such an awesome proclivity--and "desire"--to infect humans by microbes is what makes my job as an infectious disease physician so much fun. I get to battle these things!