Virus is a microscopic organism which is harmful in nature due to its ability to change its structures rapidly.
They are nonliving outside the host organism which means they don’t multiply/replicate outside the host but as soon as they infect an organism, they become living and starts reproducing inside the living cell.
The diseases caused by viruses are called viral diseases.
Viruses infect all life forms, from animals, plants to microorganisms like bacteria.
For example:- Influenza(flu), Common cold, Cough, Chicken pox, Polio etc.