STUDY / Why do some viruses never leave the body?

Even the healthiest people usually carry viruses in their bodies, which may or may not cause symptoms. In many cases, these viruses remain latent, avoiding detection and therefore not being completely eliminated.

A new study sheds more light on these hidden viruses, their prevalence, and how genetics and other factors influence the body’s ability to control them, reports Science Alert.

A research team led by scientists from Harvard Medical School analyzed data from blood and saliva samples of over 917,000 people, using three medical databases. The researchers examined genetic traces of viruses circulating in the body without turning into disease, measuring viral load, meaning the amount of viral DNA present in the body.

The analysis showed that viral load is linked to specific regions of the human genome. 82 genetic loci were identified as being associated with viral DNA load, mainly in the Major Histocompatibility Complex (MHC), a key control center of the immune system. The results suggest that each person’s genetic makeup plays an important role in how effectively they suppress viruses that remain latent in the body.

The study also identified different patterns for different viruses. The Epstein-Barr virus, for example, appears to become more prevalent with age, while the HHV-7 herpes virus decreases starting in middle age. Epstein-Barr viral load also increases in winter and decreases in summer, unlike some other viruses that remain more stable.

Link between viruses and diseases

Using Mendelian randomization, researchers also analyzed links between viruses and specific diseases. They found that a high Epstein-Barr viral load is a direct risk factor for developing Hodgkin lymphoma in adulthood. However, no direct link was found between this viral load and multiple sclerosis, although Epstein-Barr is considered a known triggering factor for it. According to researchers, this suggests that how the immune system responds is more important than the amount of virus itself in the body.

The study also examined non-genetic factors such as age, sex, and smoking. Most viruses analyzed were found more often in men than women. Three anelloviruses (a family of viruses discovered in 1997) are detected in 80–90% of the general population, although their exact link to disease remains unclear.

Researchers emphasize that the analysis was limited to DNA viruses because it was based on genomic sequencing data. This means RNA viruses, such as coronaviruses, were not included. The study, published in the journal Nature, opens the way to a better understanding of why people carrying the same viruses do not necessarily have the same risk of illness.

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