David Cox in The Guardian:
Epidemiological studies are now revealing that the number of individuals who carry and can pass on the infection, yet remain completely asymptomatic, is larger than originally thought. Scientists believe these people have contributed to the spread of the virus in care homes, and they are central in the debate regarding face mask policies, as health officials attempt to avoid new waves of infections while societies reopen.
But the realisation that asymptomatic people can spread an infection is not completely surprising. For starters, there is the famous early 20th century case of “Typhoid Mary”, a cook who infected 53 people in various households in the US with typhoid fever despite displaying no symptoms herself. In fact, all bacterial, viral and parasitic infections – ranging from malaria to HIV – have a certain proportion of asymptomatic carriers. Research has even shown that at any one time, all of us are infected with between eight and 12 viruses, without showing any symptoms.