Table 1.

Factors that May Influence Infectious Diseases Transmission

Factors that Influence Transmission	Examples from MERS-CoV, SARS-CoV, and EBOV
Virus and Host	Virus mutation: the virus may acquire mutations to replicate more efficiently and become highly pathogenic, which may result in higher levels of virus shedding for a longer period of time.
	Duration of contact with an infectious host and route of infection: in a guinea pig model of EBOV infection, animals that were infected mucosally were more infectious than those that were infected systemically. Additionally, prolonged contact with a contagious animal resulted in a higher rate of virus transmission.
	Genetic susceptibility: EBOV is known to be more pathogenic in cynomolgus macaques compared to other nonhuman primate species, such as rhesus macaques. Different nonhuman primate species also demonstrate variable susceptibility to SARS-CoV infection. Hosts that are more susceptible to disease may exhibit higher levels of virus shedding.
	Underlying medical conditions: certain conditions may decrease immunity and increase susceptibility to pathogens, leading to enhanced virus shedding. Many fatal MERS-CoV patients in the recent outbreak in South Korea had other pre-existing medical conditions, including pneumonia and kidney disease.
Environment	Air re-circulation in enclosed spaces: this would increase chances of pathogen encounter, especially those that are airborne. During the SARS-CoV outbreak, this was shown to be a factor behind super-spreading events at the Hotel Metropole, Amoy Gardens housing complex, and a flight between China and Canada. It is likely a contributing factor to the MERS-CoV outbreak since super-spreading events occurred at a hospital or medical clinic.
	Population density: increased numbers of people equally increases the chances to infect a naive host through inadvertent direct or indirect contact.
Behavior	Traditional customs and beliefs conducive to infectious disease spread: the culture of “doctor shopping,” in which patients seek medical attention from multiple doctors at different clinics/hospitals allowed MERS-CoV to spread rapidly in South Korea. Unsafe burials and traditional funerals, which involve touching and washing infectious bodies, played a role in EBOV spread in Western Africa.
	Travel and trade: the global nature of today’s world allows infectious diseases to easily bypass geographical barriers. For instance, MERS-CoV originated from the Middle East, but imported cases were reported in North America, Europe, and Asia. SARS-CoV originated in Asia, and imported cases were reported in North America, Europe, and other parts of Asia. EBOV originated from Africa, and imported cases were reported in North America and Europe.
	Knowledge and adherence to public health advice: during the MERS-CoV outbreak, a case was imported to China because a South Korean patient did not follow the recommendations of health authorities and traveled despite being a symptomatic, high-risk contact. However, China handled the imported case properly, and the imported Korean patient did not become a super-spreader. During the EBOV outbreak, some patients were able to escape quarantine, thereby increasing the likelihood of infecting others.