"The very sad news for the Monster Yamaha Tech3 team is that Jonas Folger will fly home to Germany tonight and he will not compete in the Japanese GP. He has a disease but we don't know exactly what it is, however, Jonas has been checked by Dr Charte, the MotoGP World Championship doctor, who confirmed that he is very weak and that this condition has lasted for several weeks. If there is anything good about this situation, it is that we now know why he has been so weak since the summer break. The more he trained, the worse he felt, and the peak of the problem was on Tuesday and Wednesday where he had to stay in bed because he couldn't even walk and he unfortunately had to miss the Yamaha promotion activities. For sure he will miss this round, but we don't want to start guessing as to when our rider will return. When Jonas gets back to Munich he will have a thorough check in the hospital as recommended by Dr Charte, and we will know more then. The entire Tech3 team wishes him a swift recovery and we all need Jonas back with us but at the moment there is not much that we can say or do. He will not start this race and it's 99% certain that he will not participate in the Australian round. More news will come, but for now, we just want to express our sadness and wish him a speedy recovery. We all know that he is a top rider when he is on top form."
Source : https://www.yamaha-racing.com/motogp/articles/news/2017/718476/folger-forced-to-withdraw-from-forthcoming-motogp-action