Viruses are not usually regarded as living cells. A virus 'particle' requires a living host cell in order to reproduce. Although we usually think of viruses as causing diseases, they also are an integral part of natural ecosystems controlling the size of bacterial populations by preying on them. This image was provided by Mark Young and George Rice, Montana State University and subsequently colorized is of numerous particles of a virus that infects Sulfolobus. Image copyright: Globe Pequot Press, used under license to MBL (micro*scope).