Molecular PCR tests are used to directly detect the presence of the virus, rather than the presence of the body’s immune response, or antibodies. By detecting viral RNA, which will be present in the body before antibodies form, the molecular PCR tests can tell whether someone has the virus very early on and whether they are infectious. An antibody test tells us what proportion of the population has been infected. It won’t tell you who is infected, because the antibodies are generated after a week or two, after which time the virus should have been cleared from the system. But it tells you who has been infected and who is likely to be immune to the virus.