Intense physical exertion can temporarily dampen the immune system, raising the likelihood of catching respiratory viruses. Researchers from a U.S. state lab in the Pacific Northwest published their findings in Military Medical Research.
The study followed 11 firefighters who wore heavy fire-resistant gear during a 45-minute training session. Immediately after, blood, urine and saliva samples were collected for analysis. Scientists examined more than 4,700 molecular markers, including proteins, lipids, and other metabolic byproducts. They observed shifts in molecules tied to tissue repair, hydration, energy supply, and protein and oxygen requirements of body tissues. These molecular changes point to a complex physiological response triggered by strenuous work.
Saliva samples yielded the most striking results. The oral microbiome shifted in a way that suggests the body doubled down on attacking bacteria. At the same time, several molecules linked to inflammation and antiviral defense declined. A reduced inflammatory response can ease breathing and satisfy the body’s immediate need for more oxygen, but it may also leave the system more vulnerable to respiratory viral infections. The researchers noted that this pattern aligns with experiences reported by medical professionals who treat athletes, who often become ill during intense training phases. Earlier investigations have shown that in the first days following a hard workout, the risk of acute respiratory viral infections can rise, even among those who are otherwise in good shape. This underscores how a period of heavy training can temporarily tilt the balance between immune defense and exposure to airborne pathogens.
The findings emphasize the interconnected nature of physical stress, immune function, and infection risk. They also highlight the importance of pacing training loads, ensuring adequate recovery, and maintaining strategies that support immune health during demanding athletic programs. In practical terms, teams and individuals may benefit from monitoring training intensity, prioritizing sleep, nutrition, and hydration, and considering preventive measures to reduce viral exposure during peak training periods. These insights add to the growing body of evidence that body systems respond as a coordinated unit under stress, with implications for athletes, first responders, and others who regularly engage in strenuous activity.