Up to 50 % of the human body consists of life forms without human DNA, wee need all these microbes, fungi and viruses to survive. Mostly for digestion and for the immunes system. Antibiotics and antivirals can disturb the balance of the symbiotic life forms. For example, when Penicillin became common and people started to take it as precaution, the bacteria that killed the polio virus got killed and polio became epidemic.