Virus is actually just a piece or strand of genetic information usually DNA or RNA which can be single or double stranded.
Viruses are everywhere from our surroundings to the very food we eat. We ingest, inhale and talk in millions of viruses every second. Pretty much all life on Earth has some kind of virus designed specifically to infect in including bacteria and even other viruses. Viruses are tiny very tiny you can visibly see them if fact they are so small that if bacteria had eyes even they wouldn't be able to see all of them.
A virus is a type of cell that can only multiply while inside the body of a host. When it does multiply, it takes needs from the host's body. The body's immune system then takes over and trys to drive the virus out, but as the virus is constantly multiplying, it doesn't work a lot of times. sometimes medicine is injected to either help fight the virus or stop it from multiplying.
