The struggle between virii and human is the likes of cops and robbers. It is a part of the cycle of life and death, it is yin and yang and will continue for eons.
It is the virii that seeks to exist on the cells that make up one's being and when an intruder invades our body the mechanisms of policing are put into play. Our cells send out distress signals when attacked and thanks to our wonderful communication system, action is quickly taken.
An attacker must be identified quickly before it can replicate out of control for the virus will use the attacked cell to replicate more virii. In many cases a message can be sent to central security identifying the attacker and it is then that weapons of, dare I say, "Mass Destruction" can be replicated and sent to the breached area for abrupt execution. Sounds like a good plan to me.
In some cases an attacker can't readily be identified. This happens when a particular strain has mutated, such as "influenza" and the body has yet to encounter this "new" strain and work out the tactics. This happens allot in people who obsessively stay germ free. The body needs germs to stay up to date with "virus definition files", so to speak. There is a need to know what is out there beyond the body, however one wouldn't want to overwhelm their system by wallowing in vat of germs and virii.
When an attacker is identified, the body needs to flood the affected area with an arsenal of weapons and workers or enzymes and antibodies. It takes allot of energy to pool these things together so you may feel run down while battling the "bug". You may also notice a raise in your overall body temperature. This is a good thing. It means the body is battling the enemy. Everything in moderation, though, if a body temperature exceeds 103 degrees or remains above 98.6 degrees for more than 3 days, that body should seek another body's help, preferably a body wearing plenty of white clothing and charges a small fee for a visit.....oh, and likes to golf.