The rubeola virus causes measles, an infectious disease. It spreads by direct contact with a virus-infected person or through droplets in the air. Measles is a highly infectious illness with potentially fatal consequences. No vaccine is available. Immunity to measles is achieved through repeated exposure to the virus. The immune system produces antibodies that fight off infection. Two types of antibodies are involved: immunoglobulin M (IgM) and immunoglobulin G (IgG). After acquiring immunity, blood levels of IgG rise and remain high for life. This is why someone who has been vaccinated and becomes sick with measles has higher levels of IgG than someone who has never been infected with the virus. High levels of IgG also indicate that someone has had more recent exposure to the virus and is therefore likely to be revaccinated. IgM levels in the blood drop after infection and then gradually increase as the body builds up its immunity. These antibodies are responsible for clearing the virus from the body after an initial infection. A blood test can tell whether you have been exposed to the virus and are currently immune. A negative test does not mean that you will get sick. It only indicates that you have not been exposed to the virus recently enough to build up an immune response.