Virus

A virus is a small infectious agent that replicates only inside the living cells of other organisms. Several viruses have been associated with the onset of ME/CFS including Epstein-Barr virus (which causes infectious mononucleosis / glandular fever), enteroviruses (the putative cause of many outbreaks of epidemic myalgic encephalomyelitis), and Ross River virus.

A prospective study in Australia found that 11% of people infected with Epstein-Barr virus, Ross River virus or Q fever (a bacterial infection) met the criteria for chronic fatigue syndrome six months later.

Herpesvirus
Herpesviruses are a family of DNA viruses with extremely high prevalence rates. Once a human host is infected, the infection is life-long. While generally, immunocompetent hosts are able to keep the virus in a latent state and remain asymptomatic, several of these viruses can cause symptoms if they reactivate. They can also increase the risk of autoimmune disease and cancer.

It is unclear whether herpesviruses associated with Chronic fatigue syndrome play an etiological role or are "bystanders" – opportunistic reactivations under a state of immune dysregulation. In the 1984 Incline village outbreak, Gary Holmes found that patients with what his team hypothesized was chronic Epstein-Barr had elevated antibody titers to Epstein-Barr virus, cytomegalovirus, herpes simplex and measles viruses than age-matched controls. However, the study cohort was defined as patients who had experienced excessive fatigue between January 1 and September 15.

A prospective study of 250 primary care patients revealed a higher prevalence of chronic fatigue syndrome after infectious mononucleosis (glandular fever) when compared to an ordinary upper respiratory tract infection. Anti-early antigen titers to Epstein-Barr virus were elevated in CFS patients and associated with worse symptoms.

Enterovirus
Enterovirus is a genus of positive single-stranded RNA viruses. Viruses in the enterovirus genus include coxsackievirus A, coxsackievirus B, echovirus, poliovirus and rhinovirus, though only coxsackievirus B and echovirus have been linked to ME/CFS. Person-to-person transmission of enteroviruses occurs through fecal-oral and oral-oral routes.

Enteroviruses are responsible for a range of acute infections and illnesses. They cause about 10 to 15 million infections and tens of thousands of hospitalizations each year in the United States. But acute enterovirus infections can often be mild (like a common cold) or asymptomatic when contracted.

Though normally only capable of acute infections, under certain circumstances enteroviruses can create chronic infections, and ongoing enterovirus infections have been found in ME/CFS and several other chronic illnesses in including dilated cardiomyopathy, and type 1 diabetes. Some researchers posit that such persistent enterovirus infections may be a cause of these diseases.

Effects on cellular respiration
Some viral infections may affect mitochondrial function and reduce cellular respiration. A study of poliovirus found that polio infection rapidly decreases (45% decrease) cellular oxygen consumption (and thus energy production through cellular respiration) by inhibiting succinate dehydrogenase (55% decrease in activity) and blocking mitochondrial electron transport at or between complexes I and III.