How do viruses cause disease?

A virus is an example of a pathogen. Pathogens also include bacteria, fungi or protists, and all are capable of causing infectious disease. Viruses are very small, much smaller than bacteria. They consist of a small amount of genetic material protected inside a protein shell. Viruses live and reproduce inside host cells. Once inside a host cell it rapidly makes thousands of copies of itself, eventually filling up the host cell and causing it to burst. This cell damage (and the host cell's immune response against the virus) is what causes disease.