If you think that the illness might be serious ,you should n

解析：句意：如果你不想你的病情加重,你就应该赶快去看医生.put off +v-ing,意为推迟做某事,而如果put aside为把……放到一边去干另外一件事,如果此题用put aside的话,应该为：
If you think that the illness might be serious ,you shiuld not put the illness aside but go to the doctor.