The virus test detects active infection and works best during the first seven days of symptoms. The virus can be detected in a COVID-19 swab 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 COVID-19 swab samples) and the symptoms of the illness usually recede, while the level of antibodies in the blood goes up.
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COVID-19