Rapid Antibody (IgG & IgM) testing for Viral disease

When someone gets infected with a virus like SARS covid-19. You mount on immune response and what happens there is Covid-19 is circulating in your body, your body build antibodies those antibodies attack the virus and clear the virus from your systems this is the basics of immunology.

If someone can trace covid-19 but shows no symptoms serology could be used within 3 to 10 days to determine. whether they have been infected your body first has a rise in immunoglobulin M or Ig M following that usually around 7 to 14 days later depending on the virus. You mount immunoglobulin G or Ig G. Ig G response is the later response and that is long lasting in a healthy individual. 
Ig M levels will rise before Ig G levels. You can see Ig M levels right as someone's been infected. so someone who has active covid-19 towards the middle to end of that you will start seeing Ig M rise. Ig M clears the body quicker than Ig G. So we look at Ig M to tell us that someone's recently been infected and sometimes using IgM you might be able to say that someone might still be infected than you have Ig G. Ig G is start spiking especially as Ig M Start's coming down. Ig G is what we look at to see if someone has long lasting immunity or mounted a long lasting immune response. So even if the person was asymptotic and had covid-19. If a random sampling of the population were done that person might show that they have antibodies to Covid-19. That means, they were infected with Covid-19 and they have mounted the appropriate immune response to combat the virus in their system.

When we talk about immunity to a virus or a disease.we talk more about immunoglobulin G. Because that's the immunoglobulin that lasts longer. It tells us that someone has been infected and that they have recovered from it.