The Hepatitis B virus (HBV) is a viral infection that attacks the liver; many cases are asymptomatic, other are serious, some are deadly. There is no cure. You can, and probably should, get vaccinated: that's the only real protection,
Hepatitis D is just extra bonus Hepatitis B. The hepatitis delta virus (HDV) is a 'satellite virus' and can only replicate with the aid of the HBV virus. It is stripped-down virus, sometimes called a 'defective virus' -- or more technically, a Ribozyviria -- that lacks the ability to replicate on its own, but does have a virus nucleocapsid protein. It does not, however, have the protein structures necessary to enter the host cell and to replicate, and it must take these from HBV -- or more technically, it does not have a fully functional viral envelope, having to steal three additional surface antigens from HBV.
HDV infection makes all hepatitis symptoms much worse, increasing the severity of infection and the speed at which it progresses, and increases the odds of chronic liver disease, cirrhosis, liver cancer, and liver failure. Globally, 5% of people with chronic HBV infection are also infected with HDV.
Before we understood that HBV and HDV were separate viral infections, acute joint infection was known as Lábrea fever. In the 1980s HDV was placed in the now misleadingly-named genus of Deltavirus; this is very different from the Covid-19 Delta variant, which is also sometimes referred to as the Delta virus.
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