Most people have multiple moles, and they come in lots of different shapes and sizes. Usually they’re harmless, but occasionally they could be a sign of a very serious disease: melanoma. So, when is a mole just a mole, and how does it turn into a deadly form of cancer?
SICK is a new series that looks at how diseases actually work inside our body. We'll be visiting medical centers and talking to top researchers and doctors to uncover the mysteries of viruses, bacteria, fungi and our own immune system. Come back every Tuesday for a new episode and let us know in the comments which diseases you think we should cover next.