Viral infections occur due to infection with a virus. Millions of different viruses may exist, but researchers have only identified about 5,000 types to date. Viruses contain a small piece of genetic code, and a coat of protein and lipid (fat) molecules protects them. Viruses invade a host and attach themselves to a cell. As they enter the cell, they release their genetic material. This material forces the cell to replicate the virus, and the virus multiplies. When the cell dies, it releases new viruses, which infect new cells. Not all viruses destroy their host cell, however. Some of them change the function of the cell. Some viruses, such as human papillomavirus (HPV) and Epstein-Barr virus (EBV), can lead to cancer by forcing cells to replicate in an uncontrolled way. A virus can also target certain age groups, such as infants or young children. Viruses may remain dormant for a period before multiplying again. The person with the virus can appear to have fully recovered, but they may get sick again when the virus reactivates. Viral infections include: the common cold, which mainly occurs due to rhinovirus, coronavirus, and adenovirus, encephalitis and meningitis, resulting from enteroviruses and the herpes simplex virus (HSV), as well as West Nile Virus, warts and skin infections, for which HPV and HSV are responsible, gastroenteritis, which norovirus cause, COVID-19, a respiratory disease that develops after a novel coronavirus infection that is currently causing a global pandemic.