Viruses causing diarrhoea. Coloured transmission electron micrograph of two virus types causing diarrhoea in humans. The larger spheres & tubular forms are rotavirus particles; the smaller, brighter forms are astroviruses. Rotaviruses are probably the most common viral parasites of man, causing diarrhoea in the young. The smooth outer layer of whole particles resembles the rim of a wheel. The smaller astrovirus also causes diarrhoea & vomiting, particularly in children. Identification of these very small viruses is based on their vast numbers (in faeces) & a unique 5- or 6-pointed star on the surface. Magnification: x24,000 at 6x4.5cm size.