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) within a non-enveloped icosahedral (20-sided) 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.