Hepatitis E, a faecal-oral waterborne acute viral hepatitis, occurs most frequently in epidemic outbreaks in developing countries. Hepatitis E virus (HEV) is spherical, nonenveloped and varied in size from 27 to 34 nm with a spiked surface. Experimental HEV infection in primates, molecular cloning involving cDNA libraries and recombinant protein technology indicate that the genome can be assigned to a 7.6 kb single stranded positive polyadenylated RNA. In a 5'NS-S poly A 3' molecular structure, three partially overlapping ORF have been identified. Two strains (Burma and Mexico) have been studied and although they related to caliciviruses, the genetic organization indicates that it represents different agents ranged in a new subgroup: the alpha-like viruses. HEV is an important cause of morbidity and mortality in developing countries in the 15 to 40 year age-group and although the mortality rate is low in the general population (0.5 to 3%), it averages 17 to 20% among third-trimester pregnant women. Several sporadic cases have been recently identified among children. The prevalence of anti-HEV antibodies in blood donors ranges from 2 to 3% in North Europe and USA and from 6.8% in Spain to 70% in Thailand. The average incubation period is 6 weeks. Chronic liver disease, persistent viraemia and oncogenicity have not been observed. HEV particles are identified in stool or bile by immune electron microscopy, capture immunoassay or RT-PCR.(ABSTRACT TRUNCATED AT 250 WORDS)