I’ll chime in with a bit of biology.
HDV is something called a “satellite virus”. It is a defective virus-like agent that is dependent on HBV for transmission between cells and between people. This is because it does not make its own proteins that are needed for moving between cells. Instead, it steals the HBsAg proteins from HBV to make its own outside layer, and the HBsAgs do the work of getting HDV out of cells and into the next cell. It is a really interesting virus from a scientific point of view, with its closest relatives being found in plants (in fact, its very tiny RNA genome can replicate in potato cells if we use molecular techniques to get it into the cells!).
Medically, all HDV patients are also HBV patients, but not all HBV patients are infected with HDV. I fully agree with Thomas that HBV patients should be screened for HDV. Prevention of HDV infection is the same as preventing HBV infection. As HDV uses the HBsAgs to move between cells and people, vaccinating against HBV stops HDV transmission (but this does not work after a person already has HBV). Other methods that impede HBV spread, such as avoiding direct blood contact and using safe sexual practices, also help reduce the risk of HDV transmission. Finally, there is a newly approved drug that is available in Europe and Russia, and is being tested in the USA, that can stop HDV entry in cells. It used to be called Myrcludex B, but is now called Hepcludex or bulevirtide. Finally, a number of other drugs are in development or clinical trials to help stop HDV.
I hope this helps.