Health insurance is also mandatory for all international students in the United States, so you will not be able to start studying without it.
Note that with very few exceptions, there is no system of free healthcare or subsidized health insurance schemes in the U.S., so you will probably have to take out private insurance. These insurance policies are very expensive, so it often better to extend your national health insurance for a stay in the U.S. Note that without such an extension, your national insurance will probably not cover the astronomical medical cost in the U.S. as they will probably exceed the ones from your home country.
If you have to take out an American insurance, there are many options to choose from, so you should study them carefully before you buy one. The internet is a good way to shop around because you can get many quotes quickly. You can also subscribe to Expat Offers, which will find and send you the best deals on health insurance. Your age, health history and the state you live in are factors that will affect your quote.
If you are working, there is a chance your company will offer to split the cost of the insurance with you. In many cases this is a good deal for the employee. Some companies will give you the option between more than one insurance company. Talk to your co-workers to find out which one is the best in your area. The insurance will cover you and your family as long as you remain working for the company. If you quit or lose your job, you have the right to keep the same health plan but you will probably be required to pay the full amount yourself.
Proof of insurance
When traveling in the U.S., always carry proof of your insurance with you. Otherwise you will be forced to pay a large deposit when visiting a hospital or doctor, since nobody will take the risk of giving you a treatment without a guarantee of being paid.