Zapping frozen meals in the microwave may be fast and easy, but it also can make you sick if it's not done properly.

The government has issued a new warning urging consumers to thoroughly cook frozen chicken dinners after 32 people in 12 states were sickened with salmonella poisoning.

The problem is that microwaves heat unevenly, and can leave cold spots in the food that harbor dangerous bacteria, such as E. coli, salmonella or listeria. So microwaving anything that includes raw meat, whether it's frozen or thawed, can cause problems.

Many people wrongly assume all frozen meals are precooked and only need to be warmed. It's a misconception fostered in part by foods prepared to appear cooked.

One researcher says microwaves are "great for reheating, but maybe not so good for cooking."