Another group is the protozoans, single-celled animals. These include amoebae, some of which cause a severe form of dysentery; and marine dinophytes, which can suddenly multiply in enormous numbers to produce the ‘red tides’ that make shellfish poisonous. Algae include both single-celled and multicellular types, such as seaweeds. Only one group of single-celled algae, Chlorella, has anything to do with food: it has been suggested that this should be grown in space stations to renew the oxygen in the air and to provide a raw material that could be processed into human food. Viruses are not usually classed as micro-organisms, for they are not truly alive and can only operate by hijacking living cells. They are relevant to food only insofar as they cause diseases in plants and animals.