The U.S. government is fighting to contain and slow down the spread of the coronavirus. Testing is central to these efforts. Molecular biologist and viral researcher Maureen Ferran answers some basic questions about how these diagnostic tests work—and if there are enough to go around.

Who gets tested for the virus?

Currently there are two main reasons someone would be tested for the coronavirus: having symptoms or exposure to an infected person.

The main symptoms of COVID-19, the disease caused by the coronavirus SARS-CoV-2, are fever, dry cough, and shortness of breath. These look a lot like the flu and the common cold, so it takes a physician to determine if testing for the virus is necessary.

Initially, the Centers for Disease Control and Prevention recommended testing only people with symptoms and who had potentially been exposed to the virus. But to the surprise of public health officials, several of the first people in the U.S. who tested positive for the virus had no obvious exposure. This development suggested that the virus was being transmitted locally, meaning it was spreading from person to person easily and/or that people may have been transmitting the virus without experiencing serious symptoms.

In response, on March 4 the CDC changed its recommendations to allow anyone with COVID-19-like symptoms to be tested as long as a doctor approved the request. Since the number of available tests is limited, the CDC is encouraging physicians to minimize unnecessary testing and consider a patient’s exposure risks before ordering tests.

As of writing this, there are no specific treatments available for COVID-19, but that does not mean testing is pointless. Perhaps most importantly, testing is done so that infected patients can be quarantined and the spread of the virus slowed. Another benefit of testing is that it lets public health workers build a more accurate picture of the number of cases and how the virus is spreading in the population.

What is it like to get tested?

For a patient, the process of being tested for the virus is easy and can potentially be done almost anywhere. It typically involves taking a swab from deep in a patient’s nasal cavity to collect cells from the back of the nose. The sample is then sent to a lab, where it will be tested to determine if the patient’s cells are infected with the virus. The same process is used to collect a sample from a patient who is tested for flu.