You are the commander of human body´s sentinels with clear objective: bacteria and viruses shall not pass through the defense system. Way how to deal with enemy hordes is simple. Think tactically, construct and upgrade buildings, reinforce troops, support them with special upgrades and annihilate the enemy with mighty powers.
Battlefields are situated in 17 human body organs like aorta, appendix, stomach and you will fight even in brain.
But beware! Enemies also use some tactics: Flu divides itself, Nicotinic can be invisibible and so on. Just explore and improve your fights.