A virus is a microorganism much smaller than bacteria, incapable of growing or reproducing without a living host cell. It infiltrates living cells, hijacking their biochemical processes to survive and multiply. This replication can occur accurately or with mistakes (mutations), and the ability to mutate allows some viruses to slightly alter in each host, complicating treatment efforts. Viruses are the culprits behind many common human infections and some rare diseases. Viral infections vary widely, from the common cold, often caused by rhinoviruses, to more severe illnesses like those caused by other viruses. Viruses can contain either DNA or RNA as their genetic material. Examples of DNA viruses include the simplex virus, while RNA viruses, such as HIV, have an enzyme that reverses the typical DNA-to-RNA process, enabling the virus to create a DNA copy of itself. Scientists categorize viruses into several major families based on their shape, behavior, and other traits. Among DNA viruses are families like the herpes-viruses, adenoviruses, papovaviruses (including papillomaviruses), hepadnaviruses, poxviruses, and parvoviruses. RNA viruses include families such as picorna-viruses (which include rhinoviruses), calciviruses, paramyxoviruses, orthomyxoviruses, rhabdoviruses, filoviruses, and retroviruses. Numerous smaller virus families exist within these main groups. Many viruses are host-specific, capable of infecting and causing disease only in humans or certain animals.