COVID-19 (coronavirus disease 2019) is the disease resulting from infection with a newly emerged coronavirus named SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2). Coronaviruses are a group of viruses usually found in animals. Most known animal coronavirus cannot directly infect humans, but sometimes the virus changes (mutates) and this can produce an ability to cause infection in people. Four commonly circulating coronaviruses associated with mild respiratory disease (the "common cold") with human transmission are well-known.
In 2003, the first SARS coronavirus (now called SARS-CoV) was identified in China and subsequently spread. It may have started in a bat and then spread to the civet (a small, nocturnal mammal) where it then transmitted to humans. Unlike previously identified coronaviruses, SARS-CoV was much more harmful to people. Like COVID-19, patients often presented initially with mild respiratory symptoms. Some would suddenly worsen, developing a severe acute respiratory syndrome that required critical intervention for the patient to breathe (such as placement on a ventilator). Mortality from the first SARS was estimated at about 5-10%. Unlike SARS-CoV-2, the first SARS virus did not appear to spread easily from human to human, and infection was successfully suppressed.
A related but even more dangerous coronavirus called MERS-CoV (Middle East respiratory syndrome coronavirus) outbreak occurred in 2012. While much more deadly (killing as many as 1 in 3 patients), it also did not spread easily between humans. This coronavirus may also have originated in bats, and then spread eventually to camels that subsequently transmitted infection to humans they came in contact with.
What sets the current SARS coronavirus apart (SARS-CoV-2) is its contagiousness, as it spreads readily from human to human, or from the environment to humans. Published data shows the virus can persist on surfaces and remain infective for hours or even days depending on surface and conditions, which is why disinfecting surfaces is a key recommendation. On average, someone with the COVID-19 virus is thought to infect two or more other people. This is considered a high rate of infectivity. For example, the infectivity rate of seasonal flu is typically less than 1.