That is a good question.
https://www.nytimes.com/2017/06/08/sports/nike-oregon-project-doping-allegations.html
This was also interesting:
http://www.telegraph.co.uk/athletics/2017/08/09/iaaf-consider-formal-action-against-bbc-grilling-medical-expert/
Dr. Venning repeatedly stated that letting Makwala race would “risk many more athletes not competing†due to the “very infectious and virulent†nature of the virus.
She explained: “He had taken a history, examined him, pulse, respiratory and the history from this gentleman is clear that he had symptoms similar to all the other athletes classified as having this gastro-intestinal disease.â€
Not content with Venning’s explanations, the athletes-turned-PUNDITS took it in turns to fire questions at her. Johnson questioned whether Makwala’s symptoms “could be attributed to ANY OTHER REASON as opposed to making the ASSUMPTION that it is norovirusâ€.
After Dr. Venning had AGAIN EXPLAINED how the diagnosis process had worked, Lewis queried whether the IAAF “could have checked him again to see if it was CONCLUSIVE that he was unwell to runâ€.
Paula Radcliffe also questioned whether “AT SOME POINT COULD NOT A SIMPLE BLOOD TEST HAVE BEEN DONE", despite Venning stating that blood samples were not relevant.
It turns out that Ms. Paula Radcliffe is a medical expert on hemoglobin levels, anemia, hematocrit, blood tests, OFF scores, ABP, Erythropoiesis Stimulating Agents, molecular pharmacology, pharmacokinetics, anabolic steroids, biochemistry, anatomy, physiology, genetics, endocrinology, pathology, neurology, androgen receptors, hyperandrogenism...and autologous blood transfusions.
Ms. Radcliffe also holds the world record in the marathon with a time of 2:15:25.