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Cause

Transmission
COVID‑19 is a new disease, and many of the details of its spread are still under investigation. It spreads easily between people—easier than influenza but not as easily as measles. People are most infectious when they show symptoms (even mild or non-specific symptoms), but may be infectious for up to two days before symptoms appear (pre-symptomatic transmission). They remain infectious an estimated seven to twelve days in moderate cases and an average of two weeks in severe cases. People can also transmit the virus without showing any symptom (asymptomatic transmission), but it is unclear how often this happens. A June 2020 review found that 40–45% of infected people are asymptomatic.

COVID-19 spreads primarily when people are in close contact and one person inhales small droplets produced by an infected person (symptomatic or not) coughing, sneezing, talking, or singing. The WHO recommends 1 metre (3 ft) of social distance; the US Centers for Disease Control and Prevention (CDC) recommends 2 metres (6 ft).

Transmission may also occur through aerosols, smaller droplets that are able to stay suspended in the air for longer periods of time. Experimental results show the virus can survive in aerosol up to three hours. Some outbreaks have also been reported in crowded and inadequately ventilated indoor locations where infected persons spend long periods of time (such as restaurants and nightclubs). Aerosol transmission in such locations has not been ruled out. Some medical procedures performed on COVID-19 patients in health facilities can generate those smaller droplets,  and result in the virus being transmitted more easily than normal.

When the contaminated droplets fall to floors or surfaces they can, though less commonly, remain infectious if people touch contaminated surfaces and then their eyes, nose or mouth with unwashed hands. On surfaces the amount of viable active virus decreases over time until it can no longer cause infection, and surfaces are thought not to be the main way the virus spreads. It is unknown what amount of virus on surfaces is required to cause infection via this method, but it can be detected for up to four hours on copper, up to one day on cardboard, and up to three days on plastic (polypropylene) and stainless steel (AISI 304). Surfaces are easily decontaminated with household disinfectants which destroy the virus outside the human body or on the hands. Disinfectants or bleach are not a treatment for COVID‑19, and cause health problems when not used properly, such as when used inside the human body.

Sputum and saliva carry large amounts of virus. Although COVID‑19 is not a sexually transmitted infection, direct contact such as kissing, intimate contact, and fecal–oral routes are suspected to transmit the virus. The virus may occur in breast milk, but it's unknown whether it's infectious and transmittable to the baby.

Estimates of the number of people infected by one person with COVID-19, the R0, have varied. The WHO's initial estimates of R0 were 1.4–2.5 (average 1.95), however an early April 2020 review found the basic R0 (without control measures) to be higher at 3.28 and the median R0 to be 2.79.

Virology
Severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) is a novel virus, first isolated from three people with pneumonia connected to the cluster of acute respiratory illness cases in Wuhan.  All features of the novel SARS‑CoV‑2 virus occur in related coronaviruses in nature.

SARS‑CoV‑2 is closely related to SARS‑CoV, and is thought to have a zoonotic origin. SARS‑CoV‑2 genetically clusters with the genus Betacoronavirus, and is 96 percent identical at the whole genome level to other bat coronavirus samples and 92 percent identical to pangolin coronavirus.

wikipedia.org

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