Parasite miracidium. Coloured scanning electron micrograph (SEM) of a miracidium of the parasitic fluke Schistosoma mansoni. This fluke causes the disease bilharzia (schistosomiasis). The miracidia hatch from eggs laid into the intestines and bladder of humans, which are passed in urine and faeces into sewage, and ultimately into lakes and rivers. The miracidia then infect water snails, and leave these hosts as free-swimming cercariae. These can penetrate the skin of anyone in the water, beginning a new cycle of human infection. Bilharzia is endemic in tropical countries, and can cause kidney and liver damage. It is treatable with drugs. Magnification: x400 at 6x7cm size.