Drinking as few as six can cause infection, and when they reach the gut, they rapidly break down to produce up to a million baby parasites within 10 days. The smallest nasties to be found in water, and undoubtedly the most dangerous. If you're travelling in third world countries where these diseases are prevalent, then you'd be unwise not to have been immunised. But there are other water-borne viruses which can cause diarrhoea, flu and fever. They have to be killed off either by boiling, or by chemical treatment - chlorine or iodine.
B oiling kills off all water-borne pathogenic organisms.
Trouble is, you have to maintain the water at a rolling boil for 5 minutes - an extra burden on your fuel supplies. It's further complicated by the fact that boiling temperatures decrease with altitude, dropping 5 degrees C every 5,000 feet. Apart from the resultant taste, chemical tablets work fine, but they don't clean the water, and they do need to be left long enough to work properly. The flow rate figures are variable dependent on the quality of raw water available. Suck up water from a pool of sludge, and you'll do your filter in very quickly.