Rotavirus is the most common cause of gastroenteritis in children between 3 months and 2 years of age. It is most prevalent in the winter. Rotavirus, one of the reoviruses, looks like a wheel (which ROTAtes) and possesses a double-shelled icosahedral capsid with no envelope. Its genome consists of 11 segments of double-stranded RNA. A complex double-stranded DNA genome is found in enteric adenoviruses, the third most common cause of gastroenteritis in infants and children. This organism possesses an icosahedral nucleocapsid. A paially double-stranded circular DNA genome is characteristic of hepatitis B. Its envelope contains surface antigen (HBsAg). Its capsid is icosahedral and contains the genome along with DNA-dependent DNA polymerase, which also has reverse transcriptase activity. A single-stranded circular RNA genome is characteristic of hepatitis D virus. Its envelope consists of HBsAg. The virus is defective and is able to replicate only in cells infected with hepatitis B. Ref: Ray C.G., Ryan K.J. (2010). Chapter 15. Viruses of Diarrhea. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.