The virus test detects active infection and works best during the first seven days of symptoms. The coronavirus can be detected in swabs taken from the nose or throat, even before symptoms of illness first appear. This is why people can be infectious for a few days before the onset of symptoms, which usually appear about 5 days after the virus infects. Some people have no symptoms throughout their infection but are positive on the virus test and infectious to others. After a few days of illness the concentration of the virus falls (as seen in swab samples) and the symptoms of the illness usually recede, while the level of antibodies in the blood goes up.