A passenger virus is a virus

Virus

A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

 that is frequently found in samples from diseased tissue, such as tumours, but does not contribute to causing the disease

Disease

A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...

.

Experimental demonstration of passenger status

Proving that a virus has no causative role can be difficult. Although none of the following signs is definitive, evidence that a virus found in diseased tissue is a passenger rather than a causative agent includes:

• injection of the virus into healthy animals without causing disease;
• the absence of the virus at the earliest stages of the disease;
• curing the viral infection using antiviral drug
  Antiviral drug
  Antiviral drugs are a class of medication used specifically for treating viral infections. Like antibiotics for bacteria, specific antivirals are used for specific viruses...
  
  s or vaccination
  Vaccination
  Vaccination is the administration of antigenic material to stimulate the immune system of an individual to develop adaptive immunity to a disease. Vaccines can prevent or ameliorate the effects of infection by many pathogens...
  
   with no effect on the course of the disease.

Examples

A well-established example is lactate dehydrogenase virus

Lactate dehydrogenase elevating virus

Lactate dehydrogenase elevating virus, or LDV for short, belongs to part of the arteriviridae family and the nidovirales order. Also included in the nidovirales order are the coronaviridae. The arteriviridae infect macrophages in animals and cause a variety of diseases...

, which is often found in mouse tumours. Hepatitis G virus and Chandipura virus

Chandipura virus

Chandipura virus is a member of the Rhabdoviridae family that is associated with an encephalitic illness in humans. It was first identified in 1965 after isolation from the blood of two patients from Chandipura village in Maharashtra state, India and has been associated with a number of otherwise...

 are possible examples in humans. The Duesberg hypothesis

Duesberg hypothesis

The Duesberg hypothesis is the claim, associated with University of California, Berkeley professor Peter Duesberg, that various non-infectious factors such as recreational and pharmaceutical drug use are the cause of AIDS, and that HIV is merely a harmless passenger virus...

 posits that HIV

HIV

Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...

 is a passenger virus in the etiology

Etiology

Etiology is the study of causation, or origination. The word is derived from the Greek , aitiologia, "giving a reason for" ....

 of AIDS

AIDS

Acquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the human immunodeficiency virus...

.