It seems that you are referring to hepatitis D, a very interesting virus. It’s the smallest known virus to infect humans and it’s genome only codes for one protein, HDAg (hepatitis D antigen) which can be produced in two sizes. In order to replicate it needs not only the human cell but there must also be a co-infection with hepatitis B since hepatitis D “steals” the surface antigen (HBsAg) from hepatitis B so that both viruses have identical viral membranes.

From the other discussion in this thread I see a lot of argument whether viruses are alive or not. This is not particularly interesting to any person infected with hepatitis D (and B simultaneously) since it behaves in very “alive” manner and seriously impacts the life of the infected.