An
emerging infectious disease (EID) is an
infectious disease whose incidence has increased in the past 20 years and could increase in the near future. Emerging infections account for at least 12% of all human
pathogens. EIDs are caused by newly identified species or strains (e.g.
SARS,
AIDS) that may have
evolved from a known infection (e.g.
influenza) or spread to a new population (e.g.
West Nile virus) or area undergoing ecologic transformation (e.g.
Lyme disease), or be
reemerging infections, like
drug resistant tuberculosis. Nosocomial infections, such as MRSA are emerging in hospitals, and extremely problematic in that they are resistant to many antibiotics. Of growing concern are adverse synergistic interactions between emerging diseases and other infectious and
non-infectious conditions leading to the development of novel
syndemics. Many emerging diseases are
zoonotic - an animal reservoir incubates the organism, with only occasional transmission into human populations.