Inflammation is the first reaction of the immune system to an infection or irritation. It is an attempt of the immune system to dispel invaders and repair the body. It can happen in many parts of the body and often involves the following five steps:

1. The inflamed spot becomes red.
2. The inflamed spot becomes hot.
3. The inflamed spot begins to swell.
4. The inflamed spot begins to feel uncomfortable.
5. The inflamed spot might cause surrounding organs to dysfunction.

The first four of these reactions have been known for a long time, since antiquity. Finding them is attributed to Celsus. The last one was added by Rudolf Virchow in 1858.