This is the eBook version of the printed book. This Element is an excerpt from Germs, Genes, & Civilization: How Epidemics Shaped Who We Are Today (9780137019960) by David P. Clark. Available in print and digital formats.  The crucial role of vectors in disease virulence--and the best place to focus disease prevention efforts.   If a germ hitches a ride between victims via mosquito, it matters little that the first victim is too sick to move. This may even work to the germ’s advantage. Mosquitoes can suck blood without the victim swatting them. Diseases carried between people by some other agency have little motivation to evolve mildness toward humans. The best way to control them is to kill the vectors, interrupting transmission.