Answer:
Viruses keep us alive. They form part of the body's microbiome and safeguard our health. They can be harnessed to treat illness, deliver vaccines, and diagnose infections. They're wielded as research tools to illuminate biology and disease and develop new drugs.
Viruses cause familiar infectious diseases such as the common cold, flu and warts. They also cause severe illnesses such as HIV/AIDS, Ebola, and COVID-19. Viruses are like hijackers. They invade living, normal cells and use those cells to multiply and produce other viruses like themselves.