Hundreds and possibly thousands of different types of viruses are capable of infecting humans. Viruses are obligate parasites that must enter host cells to replicate. Because of their relatively simple structures, they replicate rapidly (generally in less than a day, sometimes in 2 to 3 h) and prolifically (each infected cell produces 100–10,000 new viruses). The combination of a low fidelity of replicating their genetic information, and, with some viruses, the ability to incorporate host genes into their own genome results in an extraordinary ability to mutate.