Astrovirus particles. Coloured transmission electron micrograph (TEM) of a cluster of astrovirus particles (red). These are small viruses, measuring approximately 28 nanometres in diameter. They contain a single strand of ribonucleic acid (RNA) genetic material within a icosahedral (20-sided) outer coat (capsid). Astroviruses are a major cause of gastroenteritis but have been poorly studied because they do not grow in a culture. Symptoms include nausea, headaches and diarrhoea. Magnification: x126,000 when printed 10 centimetres tall.