Here's why:

* Viruses are not considered living organisms. They lack the cellular structures and metabolic processes necessary for independent life.

* They are obligate intracellular parasites, meaning they absolutely need a host cell to survive and reproduce.

* Viruses hijack the host cell's machinery to create more copies of themselves, essentially turning the cell into a virus factory.

Some examples of viruses include:

* Influenza virus (causes the flu)

* HIV (causes AIDS)

* Coronavirus (causes COVID-19)

* Herpes virus (causes cold sores and other infections)

* Poliovirus (causes polio)

While viruses are the most well-known example, some bacteria (like chlamydia and rickettsia) are also obligate intracellular parasites and require host cells for survival.