Infectious diseases are transmitted by viruses, bacteria, fungi or parasites, otherwise known as pathogens. Many of these pathogens evolved to infect one species exclusively, so scientists thought if the host population dies out, the parasite couldn't evolve quickly enough to infect a new host. But Brooks and Hoberg say that infectious diseases are jumping from host to host frequently, and much more quickly than expected. Their theory is that the pathogens carry ancestral DNA that allows them to swiftly make the move to a new host.