Hepatitis B and Hepatitis D virus

Hepatitis B virus, or Hep B virus for short, is a member of the hepadnavirus family; hepatitis D virus, or Hep D virus, is a deltavirus. They both cause hepatitis, or inflammation of the liver. Even though they both cause hepatitis, hepatitis D virus cannot cause the disease by itself, and needs hepatitis B virus to replicate.  

Both hep B and D viruses are enveloped, so they’re surrounded by a membrane. To make things interesting, the membrane of both viruses contains hepatitis B viral proteins - specifically, they both have a surface antigen called HBs. Beneath the membrane there is a protein shell called a capsid, which has more antigens. HB core, or HBc, is in the capsid of hepatitis B. There is also an HB envelope or HBe antigen for short, which is a variant of HBc but it’s not really part of the virus. It’s secreted, and can be found in infected individuals' serum. Delta antigen or HDAg for short is in the capsid of hepatitis D virus. 

Inside the capsid, there’s the viral genetic material. Now, hep B is a DNA virus, which means that its capsid contains partial double-stranded circular DNA, which is made of a long and short strand, so there is a part where the long strand is single stranded. And it also has DNA polymerase, which is an enzyme with DNA- and RNA-dependent activity, meaning it can convert DNA to RNA and vice versa. On the other hand, hep D is an RNA virus, so its capsid contains single-stranded circular RNA in a rod-like folded structure, which is why host cell enzymes can use it as double-stranded DNA.

The main source of hepatitis B virus is blood, but it can also be found in other bodily fluids like milk, amniotic fluid, vaginal secretions and semen. So, routes of transmission include: sexual contact; contaminated blood, either following transfusions or injections with contaminated needles, the latter being more common in people who use intravenous drugs. The virus can also be passed from an infected mother to the baby during childbirth. Rarely, during the pregnancy the virus can pass through the placental barrier, which is a kind of a very thin wall that brings the mother’s and fetus' blood very close, and allows them to exchange some substances like oxygen, IgG antibodies, waste products and unfortunately certain microbes. More commonly the virus can pass from an infected individual to the child during birth because of the close contact between the individual's blood and secretions with the child. It is still not certain if C section can protect against this kind of transmission. Hepatitis D virus spreads the same way, but it only causes disease in individuals with an active hepatitis B infection.