Dear NeptuneJ,
HDV can either infect a person at the same time s/he contracts HBV if the source of the transmission was HDV+, or it can “super-infect” during a second exposure of an HBV+ positive person to second “donor” (nerd-speak for the person from whom a viral infection was acquired) who is HBV+/HDV+. HDV can be fairly infectious because it can be at very high levels in some people’s bodies. The risk of becoming HDV+ depends on the frequency of it in the community where a person is. A good reference is the US CDC site on HDV What is Hepatitis D - FAQ | CDC. An HDV endimicity map (ie, a map showing where HDV is most commonly found) is at: Hepatitis D epidemiology and demographics - wikidoc. Even in highly endemic regions, only a minority of HDV+ folks have HDV, but HDV is even more under-diagnosed than HBV is, so our data have substantial limitations.
Note that actions that block HBV infection also block HDV infection, so thankfully people are not defenseless! See my above post for details.
Your feeling that many people who have died from HBV liver cancer may also have had HDV is right. HDV is badly under-diagnosed, and it makes HBV disease worse, including liver cancer. Fortunately, drugs for HDV are becoming available (see my post above).
Hope this helps.
John.