Herpes simplex viruses. Coloured transmission electron micrograph (TEM) of a section through a cell infected with herpes simplex viruses. The viruses (yellow spheres) are accumulating in the cells inner nuclear membrane and have a core of DNA (deoxyribonucleic acid) genetic material. These are herpes simplex type I viruses, which are associated with oral cold sores. Herpes simplex is one of the most common viral parasites in humans. They cause small, fluidfilled blisters which may persist for weeks and, if touched, can spread the virus. Antiviral drugs may be helpful. Negatively stained. Magnification unknown.