All residents of Belgium, Belgian nationals or otherwise, are required by law to be covered by a health insurance policy.
Obviously, it is also in your own interest. Without insurance coverage, the costs of medical care can quickly spiral out of control. So, make sure to subscribe to health insurance, as it is not taken care of by your institution. On average, 60-75% of the official rates of (Belgian) medical expenses are reimbursed.
These are general guidelines. The situation may be different depending on your status, in combination with the regulations in your home country. Always check with your health insurance company in your country before departing to Belgium.