In addition, they emit waste materials (including poo). However, viruses do not exhibit all of these properties. Viruses are unable to move, develop, turn nutrients into energy, or expel waste. However, viruses do replicate, infecting humans and causing diseases. Virus replication requires the assembly of new virus particles by rearranging parts of themselves. This results in a mass that is less dense than its components and so can float free in water. Viruses are responsible for some very serious illnesses. Some viruses cause illness only when their usual host is infected with another virus or bacteria at the same time. For example, someone infected with the herpes simplex virus (HSV) may experience only mild symptoms or no symptoms at all. If they happen to be around someone who is infected with the varicella-zoster virus (VZV), however, they could catch the disease from this secondary infection. Other viruses, such as HIV, cause illness or death regardless of whether their host is also infected with something else. HIV destroys certain cells in your body, especially those involved in immune function. This allows the HSV to come out of its hiding place and grow back again. Although HIV does not appear on any sewage treatment plant list of contaminants, it is still considered toxic to humans. When you view viruses through a microscope, they appear as tiny balls of protein and DNA.