Viruses are the smallest microbes-only a few hundredths of a micron-and they are not truly alive, because they are not fully functional. They are parasites of individual cells and cannot do anything by themselves. Most are very specific and limited in the type of cell they can infect, and the only ones in water that are of interest to humans are those that come from human sewage. Their numbers in contaminated raw waters are usually about one per cent of the fecal coliform count (see below, in “bacteria”). They are too small and too few to cause any mechanical or maintenance problems for filters; the only “problem” they cause is disease. They are only a minor problem in developed nations with good sewage and water treatment practices, but rotaviruses in particular are the #1 cause of human death and misery world-wide: infant diarrhea caused by rotaviruses in the drinking water accounts for fully half of all human deaths on Earth each year.