" The main problem facing a parasite over the long term, Burnet noted, is the issue of transmission: how to spread its offspring from one individual host to another. Various methods and traits have developed toward that simple end, ranging from massive replication, airborne dispersal, environmentally resistant life-history stages (like the small form of C. burnetii), direct transfer in blood and other bodily fluids, behavioral influence on the host (as exerted by the rabies virus, for instance, causing infected animals to bite), passage through intermediate or amplifier hosts, and the use of insect and arachnid vectors as means of transportation and injection. "

― David Quammen , Spillover: Animal Infections and the Next Human Pandemic