Janse van Rensburg, Dina ChristinaJansen van Rensburg; AudreyBoer, Pieter HenkSerero, PreciousBotha, TanitaSchoeman, MarleneViljoen, Carel Thomas2026-01-272026-01-272026Janse van Rensburg, D.C., Jansen van Rensburg, A., Boer, P.H. et al. 2026, 'Illness is common in trail runners during training: a prospective cohort study', Journal of Science and Medicine in Sport, doi : 10.1016/j.jsams.2025.12.008.1440-2440 (print)1878-1861 (online)10.1016/j.jsams.2025.12.008http://hdl.handle.net/2263/107616DATA AVAILABILITY : The data obtained in this study is available upon reasonable request.OBJECTIVES : Trail runners often experience illness during event preparation. We aimed to prospectively determine prevalence, incidence, clinical characteristics, severity (time-loss, burden) and associated illness risk factors amongst South African male and female recreational trail runners of different ages over a 30-week training period. DESIGN : Prospective cohort. METHODS : Illness and training data from 152 trail runners (120 males, 32 females) were collected every 14 days over 30 weeks using the updated OSTRC-H questionnaire. Illness prevalence (%), incidence (I: per 1000 trail running exposure-days), incidence ratio (IR), severity [% time-loss; illness burden (IB: days lost/1000 trail running exposure-days)], and associated risk factors are reported. RESULTS : Over 30 weeks, 52 % of runners reported illness, with a mean two-week all-illness prevalence of 10.1 %. The overall illness incidence was 7.2 (95 % CI: 5.7–9.2). The winter-to-summer IR was 1.6 (p = 0.022). Illness incidence in females (I = 9.7) was higher than in males (I = 6.6), with a female:male IR of 1.5 (p < 0.05). Most illnesses affected the respiratory (64.3 %), digestive (15.9 %) and musculoskeletal (5.7 %) systems. Moderate illness severity (8–28 days time-loss) occurred in 29.3 % of runners. The overall IB was 43.2 days lost. BMI ≥ 25 kg/m2 was associated with illness, but no significant illness risk factors were identified via Poisson regression. CONCLUSIONS : During a 30-week training period, 52 % of runners reported illness, with a higher incidence in females. The respiratory tract accounted for > 60 % of illnesses. One in three trail runners experienced > one week time-loss. A higher BMI > 25 kg/m2 was associated with illness. Further research is needed to identify risk factors and develop prevention strategies. PRACTICAL IMPLICATIONS • Prioritise respiratory health monitoring • Screen for high-risk profiles early • Tailor strategies for females and higher-BMI runners • Inform trail running guidelines HIGHLIGHTS • Trail runners show an illness prevalence of 10.1% during training. • Females experience significantly higher illness incidence than males. • Illness mainly affects the respiratory system of trail runners during training. • Approximately one-third of trail runners suffer from moderate to severe illness, resulting in > 8 days of time-loss in a 30-week training period. • Trail runners with a higher BMI (> 25 kg/m2) are more susceptible to illness.en© 2025 Published by Elsevier Ltd on behalf of Sports Medicine Australia. This is an open access article.PrevalenceRisk factorsTime-lossRespiratory infectionsIncidenceArticle