Emerging Infections

Infectious diseases, whether caused by bacteria, viruses, or parasites, are complex, dynamic, and constantly evolving. Some emerge or reemerge in human populations as they cross the species barrier from animals to humans, and once they have infected humans they may be asymptomatic or cause disease. If they cause disease, they may maintain their virulence or decrease in virulence with further passage through human populations. While some of these infectious agents transmit easily from human to human causing epidemics or pandemics, others may not be transmissible, but continue to sporadically infect humans as zoonotic infections. Still others, like HIV infection, may eventually become endemic infectious diseases in humans.