It develops an immune reaction to its own survival instinct.
Allergen-A foreign substance that provokes an immune reaction or allergic response in some sensitive people but not in most others.
The combination of the infection and the immune reaction to it can cause headache and a fever, as well as more severe symptoms in some cases.
Primary encephalitis is caused by direct infection by the virus, while secondary encephalitis is due to a post-infectious immune reaction to viral infection elsewhere in the body.
Anaphylactoid (meaning "anaphylactic-like") reactions are similar to those of true anaphylaxis but do not require an IgE immune reaction.