Virus on slide. Computer artwork showing an enlarged virus (blue) on a light microscope slide. Reflections of virus structures are seen at top left. Viruses contain a core of genetic material, either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid) surrounded by a protein coat, or capsid, and sometimes an outer glycoprotein envelope. Once inside a host cell, viruses hijack the internal machinery to replicate themselves, before releasing themselves to infect further cells. In reality, most viruses measure between 10 and 300 nanometres and are too small to be seen with a light microscope.