Emerging infectious disease

An emerging infectious disease (EID) is an infectious disease whose incidence has increased recently (in the past 20 years), and could increase in the near future.^[2][3] The minority that are capable of developing efficient transmission between humans can become major public and global concerns as potential causes of epidemics or pandemics.^[4] Their many impacts can be economic and societal, as well as clinical.^[5] EIDs have been increasing steadily since at least 1940.^[6]

For every decade since 1940, there has been a consistent increase in the number of EID events from wildlife-related zoonosis. Human activity is the primary driver of this increase, with loss of biodiversity a leading mechanism.^[7]

Emerging infections account for at least 12% of all human pathogens.^[8] EIDs can be caused by newly identified microbes, including novel species or strains of virus^[9] (e.g. novel coronaviruses, ebolaviruses, HIV). Some EIDs evolve from a known pathogen, as occurs with new strains of influenza. EIDs may also result from spread of an existing disease to a new population in a different geographic region, as occurs with West Nile fever outbreaks. Some known diseases can also emerge in areas undergoing ecologic transformation (as in the case of Lyme disease^[10]). Others can experience a resurgence as a re-emerging infectious disease, like tuberculosis^[11] (following drug resistance) or measles.^[12] Nosocomial (hospital-acquired) infections, such as methicillin-resistant Staphylococcus aureus are emerging in hospitals, and are extremely problematic in that they are resistant to many antibiotics.^[13] 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 EID are zoonotic,^[4] deriving from pathogens present in animals, with only occasional cross-species transmission into human populations.^[14] For instance, most emergent viruses are zoonotic^[4] (whereas other novel viruses may have been circulating in the species without being recognized, as occurred with hepatitis C^[15]).