Cryptosporidium is a tiny parasite which is increasingly showing the ability to avoid disinfection in water supplies. It is regularly found in the intestines of cattle and sheep, and is spread through contaminated water or food.
It can cause gastrointestinal illness, characterised by fever, abdominal cramps and diarrhoea that can last for weeks in humans, though most recover in a week.
It is classified as a protozoan, and can cause infection in very small numbers in contrast to Salmonella infections, for example, which require hundreds of cells to have an impact.
Its effects can be more serious among older people, the very young and those whose immune system is suppressed, especially those with AIDS. In such cases, cryptosporidium can be fatal.
The number of cases in the developed world has been increasing, while scientists are particularly concerned with indications that the parasite, or its eggs, are increasingly showing the ability to evade standard chlorination and filtration - the kind of which is used routinely in most public water supplies and swimming pools.