Hepatitis B virus is capable of penetration of blood-testis barrier and also integrate into their genome (the genetic material of an organism). HBV can induce chromosomal aberrations (a missing, extra, or irregular portion of chromosomal DNA). This could lead to hereditary defects in male germinal cells and irregularities in spermatogenesis (sperm development).
In every body cell include sperms, there are smaller parts called organelas. One of them is mitochondry which is responsible for energy of sperm. HBV protein can change the potencial on mitochondrial membrane which leads to loss of energy. Patients infected with HBV have decreased motility of sperms. There is lower possibility of pregnancy in natural way.