Viruses are obligate intracellular parasites. This means that they can only replicate inside the cells of other living organisms. Viruses do not have their own metabolism or the ability to reproduce on their own. They must infect a host cell in order to use the host cell's machinery to replicate.

Here are some other basic characteristics of viruses:

* Viruses are very small. They range in size from about 20 to 300 nanometers (nm). A nanometer is one billionth of a meter.

* Viruses are made up of a protein coat (capsid) that encloses a core of genetic material (RNA or DNA). The capsid is made up of multiple protein subunits called capsomers.

* Viruses can infect all types of organisms, including animals, plants, fungi, and bacteria.

* Viruses are responsible for a wide range of diseases, including the common cold, flu, AIDS, and Ebola virus disease.

Viruses are a major public health threat. They are constantly evolving and changing, which makes it difficult to develop effective vaccines and treatments.