When a foreign substance is introduced, the body releases larger amounts of histamine to the site of infection.
No, It doesn't. you are usually quarantined for 10-15 days or so as the body's immine system deals with the virus. The body then retains a 'memory' of the viral coating of the particulat strain and will be better able to deal with the virus if returns. You can now get a vaccine shot if you are in a high risk group.