In most cases, it’s impossible to pinpoint how someone caught SARS-CoV-2, the coronavirus that causes Covid-19. It is too small to see with the naked eye, so its infectious journey can’t be tracked from one person to another. That leaves scientists and health authorities gleaning the most likely transmission pathways from the available evidence and what’s known about how other respiratory pathogens spread. Close personal contact with infected people via mouth and nose secretions was thought to be the dominant mode of infection. But emerging data have also implicated the tiny particles emitted when an infected person speaks, sings or even just breathes normally. Here’s an explanation of the established route of contagion and other pathways under investigation.
These spatters of virus-laden liquid of varying sizes, expelled like ballistic missiles from the mouth and nose of an infected person, are thought to be the main route. A cough can disperse virus particles 4-to-5 meters (13-16 feet) and a sneeze can project them as far as 8 meters away, depending on humidity, temperature and airflow. The droplets can cause infection by falling into an eye, nose or mouth, by being inhaled or getting stuck on a hand and transferred to one of these entry sites.
Spending at least 15 minutes in close contact (less than 2 meters) with an infected person, or even briefer periods with someone who is coughing or sneezing, is associated with higher risk for transmission. Patients may harbor potentially infectious SARS-CoV-2 virus in their saliva, stool and urine as long as 15 days after falling ill, researchers in South Korea showed in a study of five patients in July.
The virus can be highly stable in favorable environments, lingering for weeks in near-freezing temperatures. At room temperature, it can survive as long as 24 hours on cardboard, 48 hours on stainless steel, and 72 hours on plastic, one study found. Another found it survived and remained infectious for 96 hours, or four days, on a surface with protein traces, such as mucous. Standard disinfection kills it though, as does sunlight. Dry air, on the other hand, appears to favor transmission.