Virus is a non-living infectious agent, which can use the body cells to complete reproduction and proliferation, unlike bacteria or fungi, it can't do this on its own, and must find the appropriate host cells to grow and reproduce.

When the blood-borne viruses reach a certain level, they can be detected in the infected person's blood. These viruses can move from person to person by blood, in some cases, they can also spread through the body fluids, such as semen and breast milk. The blood-borne virus can also be passed by intravenous drugs use, as well as transmitted from the mother to the child, before, during or even after birth.

For human health, the most important blood-borne viruses are HIV, hepatitis B virus and hepatitis C virus; these viruses can persist in the body's blood for a long time and even for life. Researchers also found a brief appearance of certain viruses in individual blood, but in general these viruses were not persistent and were not considered to be significant blood-borne pathogens. Any infectious agent with a blood-borne, or "viraemic" phase has the potential for blood borne transmission, so this may be very important for blood transfusions.

For many infectious diseases, the virus infection period will continue until the body's immune system starts to kill all infected cells and make the infection to be cured; For some viruses, including Zika and Dengue, the virus infection period will last for several days, and for other viruses such as hepatitis B virus, hepatitis C virus and HIV, the virus infection will continue to exist.

The treatment of blood-borne diseases aims to block the replication of the virus. If the blood-borne virus can not be detected in the patient's blood, then generally the patient is considered as not contagious.