The person in the versinity of the character has measles and sneezes causing pathogens to become airborne and enter through the characters nose.
Measles travels down the throat and into the blood stream. The Body realizes something is wrong and sends white blood cells to go and fight the virus. The character starts to get a fever as it is trying to slow down the virus and maybe kill some of the pathogens.
After 10-14 days the characters snot starts to become yellow because of the dying white cells, the character also has a rash, cough, runny nose, inflamed eyes and sore throat.
B cells identifies the virus, each b cells has antibodies which carries cell membrane. Each Pathogen has a different marker on them so the b cell identifies that and match's its antibodies and locks onto like a lock and key. Once locked on the b cell sends out thousands of matching antibodies to attack the pathogens.
The B cells start to clone themselves to start an attack on the Measles virus. Once the body finally defeats the virus you start to feel better. The cells start to slowly fade away until there aren't many left but these ones become memory cells.
Once there is memory cells in your body that means if the virus comes into the body you will be ready to fight it off. Your body is quicker at locating what virus is then the virus is eliminated quicker meaning that you are immune.