Herpes Simplex Virus (HSV) causes cold sores on the lips or similar blistering of the genitals. Transmission is usually by direct skin-to-skin contact when sores are present.

Infection with herpes simplex is life-long as the virus becomes dormant within nerve cells that are connected to the site of infection. Reactivation of the virus occurs when the immune system is weakened leading to a reoccurrence of the symptoms.

Herpes Simplex Virus is a very well studied virus as it is easy to grow in the laboratory. Viruses cannot replicate outside of animal cells, so scientists grow cells in plastic bottles, fed by a liquid medium that contains all of the necessary nutrients for the cells to survive. These are then infected with virus to study how it exploits the cells machinery to make copies of itself. To see what the virus is doing inside the infected cells, they are embedded in a plastic resin and cut into very thin sections that can be viewed in the electron microscope.