There are three main types of infectious agents, namely patients with infectious diseases, people with latent infections, and carriers of pathogens. Sources of infection are an essential part of infectious diseases. Sources of infection can be human or animal, and usually carry pathogenic microorganisms and are able to continuously excrete pathogenic microorganisms and infect others, which ultimately leads to outbreaks of infectious diseases. For example, in the case of novel coronaviruses, the source of infection includes asymptomatic infected persons in addition to patients. An asymptomatic infected person, although asymptomatic, can continuously excrete the novel coronavirus and infect others through airborne, aerosolized, or direct contact transmission.