The spectrum of exertional heat illnesses (EHI) ranges from relatively mild conditions such as heat cramps and parade syncope to more serious heat exhaustion, heat injury and heatstroke. When recognized and treated promptly, many heat illness casualties can return to duty in just a few days. However, heatstroke may lead to extended hospitalization, need for rehabilitation and reconditioning, and — in the worst-case scenario — death. In the past 18 years, more than 30 Army service members have died due to heat-related illness. Responsibility for preventing heat illness lies with both the individual Soldier and the chain of command.Heat stress is the result of environmental, mission-related and individual risk factors. Environmental risk factors include air temperature, humidity, solar radiation and air movement, which is measured using the wet-bulb globe temperature index. Environmental risk factors are mitigated by altering the time of day of an event or, if that is not possible, modifying mission-related risk factors such as event duration or time standards. The mission-related risk factors include work intensity, clothing worn, equipment carried and duration of the activity. During routine training, these can often be modified to mitigate heat stress; but in some instances (e.g., operational missions, testing or training for special skill badges or tabs), they cannot be modified. Individual risk factors are about the Soldier, including his or her physical fitness, acclimatization status, any medication and supplement use, and existing illness.A risk factor that has received additional attention lately is the existence of concurrent or recent viral illness. Viral illness may cause fever, which is an increase in baseline body temperature. Coupled with the increase in body temperature during exercise, this may increase the risk of heat illness. Viral illness augments the body’s normal heat response due to exertion, increasing heat strain due to the combined effects of fever and exercise. Well-acclimated and well-conditioned individuals who are otherwise low-risk may develop EHI or exertional heatstroke (EHS) if they have a recent or current infection. Upper respiratory infections (specifically those that are viral in nature) in particular have been implicated in a large number of EHI and EHS cases. A case report from the U.S. Army Research Institute of Environmental Medicine reported that an isolated local infection with inflammation has been shown to increase overall risk for heat illness.The novel coronavirus SARS-COV2 (COVID-19) is a highly infectious viral infection causing systemic inflammatory response with symptoms of fever and respiratory compromise. In the young, healthy population, it may remain asymptomatic for as many as 80 percent of patients throughout the course (approximately 14 days); cause symptoms as mild as fever, wheezing and diarrhea or severe enough to require hospitalization for support; or lead to death. Even in more susceptible populations (elderly and immunocompromised), it is asymptomatic for the initial several days of infection, yet remains contagious during this period. With the close proximity of working and living conditions in military members and trainees, it is imperative to monitor for signs of infection and isolate these individuals rapidly to prevent spread. Those with high-risk contacts associated with COVID-19-infected individuals should be identified and quarantined to limit further transmission.The viral nature of COVID-19 as well as systemic inflammatory response and associated fevers make it a significantly concerning risk factor for heat illness even in well-conditioned, well-acclimated athletes. The virus predominantly affects the lungs (lower respiratory tract), causing shortness of breath and hypoxia, which may also increase susceptibility to EHI. Efforts should be made to identify individuals with new shortness of breath or new exercise limitations, as these may be evidence of otherwise asymptomatic illness which may cause severe or even life-threatening EHS if infected personnel continue to train. As such, COVID-19 represents an additional risk factor for EHI and EHS, which must be considered and mitigated by leaders for any populations participating in exertional training, particularly in hot or arid environments.At Fort Benning, Georgia, a Soldier who is diagnosed with a respiratory or viral infection is placed on a limited duty profile for seven days. They cannot participate in any maximum-effort or timed events, but can partake in submaximal intensity training while the profile is in effect. As there may be latent but as yet undefined effects of COVID-19 infection, leaders and clinicians should consider a similar policy for Soldiers who are returning to duty after isolation to mitigate the risk of EHS.No matter the cause of an EHI event, prompt recognition and treatment are the cornerstones of the initial response. More in-depth information on the prevention and treatment of EHI can be found in Technical Bulletin Medical 507, Heat Stress Control and Heat Casualty Management, and in TRADOC Regulation 350-29, Prevention of Heat and Cold Casualties. Detailed medical treatment algorithms and other information can be found on the Warrior Heat- and Exertion-Related Event Collaborative website at https://www.hprc-online.org/resources-partners/whec.