Bacteria has contaminated the water near the teenagers bed.
The boy drank the infected water from his room.
First the bacteria has to penetrate the non-specific, or innate, immunity barriers. The non-specific barriers include the skin and mucous membranes like the lining and hair in the nose or the lining in the stomach.
Once the bacteria, or antigen, gets passed the first line of defense, it has to avoid the lymphocytes, or white blood cells, that are involved in the specific, adaptive immunity. Because he is a teenager, his lymphocytes will have a better chance of defeating the foreign material since he has been exposed to more diseases longer than younger children.
The lymphocytes include B cells and T cells. The B cells secrete antibodies into the infected body fluid to ambush the antigen. The T cells can help kill the foreign material, or signal for complement proteins and phagocytes. The complement proteins and phagocytes cells destroy the bound microbes, killing the bacteria.
The boy may experience some pain during the time the immune system is fighting the bacteria due to inflammation at the site of the infection. The white blood cells, B and T cells, can secrete chemicals to kill the foreign material; The release of the chemicals increases the blood flow to the infection site, and can result in redness and swelling.