Generate a sentence that explains what a virus is.
A virus is a microscopic infectious agent that reproduces inside the cells of living organisms and can cause a wide array of diseases in its host.
Complete the following task based on the instruction and optional input provided.### Instruction:
Generate a sentence that explains what a virus is.

### Output:
A virus is a microscopic infectious agent that reproduces inside the cells of living organisms and can cause a wide array of diseases in its host.