Nordic skier Krista Pärmäkoski was in excellent running condition during the early training season of 2025, having achieved a personal best in the 5,000 meters with a time of 17:05.52. Leading up to the competition season in November 2025, her physical performance tests indicated strong results, and she started her season competitively in Ruka with respectable placements. However, soon after, her performance deteriorated unexpectedly, leading to concerns about her health and condition.

In early December 2025, Pärmäkoski contracted COVID-19. While the acute phase of the illness resolved in less than a week, she struggled with persistent symptoms as she prepared for the Tour de Ski at the end of the year. She described this experience as a troubling mystery, particularly noting the unusual respiratory congestion that persisted longer than expected. Pärmäkoski initially believed this lingering issue was standard post-viral mucus but found it to be a major hindrance during her racing.

Ultimately, after recognizing that the congestion was affecting her performance significantly, Pärmäkoski made the decision to withdraw from the Tour, highlighting the ongoing challenges elite athletes face in recovering from COVID-19 and the mystery around its long-term effects, which can vary greatly from person to person.