Health Insurance markets sometimes include mandates: requirements that everyone obtain health insurance. For example, the Affordable Care Act (aka Obamacare) included health insurance mandates for individuals and for large employers. Many health insurance policy proposals involve some sort of mandate. This video explains some of the economics behind a health insurance mandate, and shows how an insurance market can fall apart without one.
Click on the Everyday Econ image below, or this link to see the video.