Emerging viruses include those newly detected in humans and those increasing in incidence. New respiratory viruses may emerge through genetic mutation (e.g., via antigenic drift or shift) or through zoonotic spillover from animal reservoirs.
A well-documented example is SARS-CoV-2, which is believed to have originated from a zoonotic spillover event—potentially involving bats or intermediate hosts such as pangolins. In such cases, viral mutations may enhance the ability to bind to human receptors, enabling cross-species transmission.
Several other viruses pose potential pandemic threats. Examples include:
While most avian influenza viruses do not infect humans, some strains have caused sporadic infections. For instance, H5N1 has been transmitted to humans from poultry, although there is currently no confirmed evidence of sustained human-to-human transmission. These viruses remain a concern due to their potential to acquire mutations enabling wider transmission.
Coronaviruses have a high frequency of genetic reassortment and large animal reservoirs, increasing the risk of emerging variants. Historical outbreaks of SARS-CoV-1 and MERS-CoV, both thought to originate in bats, highlight the zoonotic risk. MERS-CoV continues to circulate in camel populations and has a high case fatality rate. Although human-to-human transmission is limited, viral recombination could enhance its transmissibility.
Although not newly discovered, human metapneumovirus is increasingly recognized as a major cause of respiratory illness in children, older adults, and immunocompromised individuals. Infections can be severe. There are currently no licensed vaccines or monoclonal antibodies, though candidates are in clinical trials.
Hantaviruses are zoonotic pathogens found in rodent populations. They can cause severe respiratory illnesses such as Hantavirus Pulmonary Syndrome. Transmission occurs through contact with infected rodents. There is no evidence of human-to-human transmission.
