PURPOSE: To investigate the relationship between sport training and the risk of common illnesses: upper respiratory tract and pulmonary infections (URTPI), muscular affections (MA), and all-type pathologies in highly trained swimmers. METHODS: Twenty-eight French professional swimmers were monitored weekly for four years. Training variables included 1/in-water and dryland intensity levels: low-load, high-load, resistance, maximal strength, and general conditioning (expressed as the percentage of the maximal load performed by each subject, at each intensity level over the study period); and 2/training periods: moderate, intensive, taper, competition and post-competition. Illnesses were diagnosed by a sports physician using a standardized questionnaire. Mixed-effects logistic regression analyses were used to model odds ratios for the association between common illnesses and training variables, adjusted for sport season, semi-season (summer or winter), age, competition level, sex and history of recent events, while controlling for heterogeneity among swimmers. RESULTS: The risk of common illnesses was significantly higher in winter months, for national swimmers (for URTPI), and in cases of history of recent event (notably for MA). The odds of URTPI increased 1.08 [95% confidence interval (CI) 1.01-1.16] and 1.10 (95% CI 1.01-1.19) times for every 10% increase in resistance and high-load trainings, respectively. The odds of MA increased by 1.49 (95% CI 1.14-1.96) and 1.63 (95% CI 1.20-2.21) for each 10% increase in high-load, and general conditioning trainings, respectively. The odds of illnesses were 50-70% significantly higher during intensive training periods. CONCLUSION: Particular attention must be paid to illness prevention strategies during periods of intensive training, particularly in the winter months or in case of recent medical episode.