If you get sick after your medical exam but before the insurance goes into effect, the health insurance company has the right to call it a preexisting condition. Typically, a preexisting condition is one for which you have sought or received medical care prior to be insured. And symptoms, not treatment, are the determining factor. If you displayed symptoms of an illness before the insurance goes into effect, then conditions existed. This can be a tricky situation, because you may not display symptoms, such as having the condition discovered during a routine exam. In this case, though, you still risk being denied coverage due to a preexisting condition because you sought medical attention, a term that is often used in health insurance policies to define preexisting conditions.
Keep in mind that it will be up to the insurance company whether to deny the claim or not. For some conditions, the symptoms are not necessarily going to cause the illness or injury to be preexisting. To decide what will be allowed to pass and what will not, read through the policy carefully, looking for specifically excluded conditions.
If you are part of a group health insurance plan, you will probably have less trouble with this issue. Group health plans are designed to give equal treatment to all members, absorbing the risk of preexisting conditions rather than having the option of excluding people because of them. Because of that group plans are quite a bit more flexible than private health plans, and much less choosy about who is allowed to join the plan.
One thing you should not do is try to hide the fact that the condition has emerged. If you do that and then it later comes to light that you didn't inform the insurance company, you risk losing your health coverage completely on the grounds of fraud and misrepresentation.