Here's why:

* Nucleic acids (DNA or RNA): Viruses use genetic material, either DNA or RNA, to encode their instructions for replication.

* Proteins: Viruses have a protein coat (capsid) that protects their genetic material and allows them to attach to and enter host cells.

* Lipids (sometimes): Some viruses have an additional outer envelope made of lipids, which they obtain from the host cell membrane during assembly.

* Infection of living cells: Viruses are obligate intracellular parasites, meaning they can only replicate inside living cells. They use the host cell's machinery to make copies of themselves.

Key Points:

* Viruses are not considered living organisms because they lack the ability to reproduce independently and require a host cell.

* They are very small, much smaller than bacteria, and can only be seen using an electron microscope.

* There are many different types of viruses, some of which cause diseases in humans, animals, and plants.