Infectious diseases are commonly spread through the transfer of bacteria, viruses or other germs from one person to another. This can happen when an infected person touches, kisses, coughs or sneezes on someone who isn’t infected.
Deeper dive
Scientists mostly agree that SARS-CoV-2, the virus that causes COVID-19 disease, is most commonly spread when an infected person breathes out droplets and very small particles that contain the virus and hang in the air.
Sometimes the droplets can fall on surfaces and linger. If an uninfected person touches the contaminated surface, they could become infected with SARS-CoV-2. But most evidence suggests this is not the primary way that COVID-19 disease spreads. Rather the most common route is through aerosolization (particles lingering in the air after an infected person breathes) or through coughing, sneezing, kissing or close personal contact.
The formula for COVID-19 being successfully transmitted to another person is related to the amount of a person’s exposure to the virus, multiplied by the amount of time someone is with the infected person, he says. The more time someone spends with someone exposed, the greater the risk of becoming infected. Anyone infected with COVID-19 can spread it, even if the person has no symptoms. Data suggests that around a quarter to half of infected individuals have no symptoms.
See this WebMD story for more details on how COVID-19 spreads.