During illness, the purpose of a fever is to inhibit the production of white blood cells.
When someone felt ill it means that something went wrong. The system is not working properly as it is expected to a normal one. This happens because the booster of the immune system is prevented. They cannot reach to the minimum number they are expected to move and do their function.
They are outnumbered by an unidentified organism that is entering to a particular system. And the system is not working as it is expected.Then, later on resulted to sickness or inconvenient to the part of the host.