Because of dosage. If you are exposed to a small amount of viral mass your natural immune system can deal with it before the virus has multiplied to the point w

Because of dosage. If you are exposed to a small amount of viral mass your natural immune system can deal with it before the virus has multiplied to the point where you get sick. If you're confined to a small space where the virus is prevalent you're getting a big dose and your immune system has to kick into high gear and produce antibodies.