So I put viruses into people not cancer but the problem with cancer is that when it grows it can get so big that it pushes its way through into our blood stream when it is growing.
One or two of the cancer cells might get knocked off by the movement of blood and these are then loose in our blood until they stick somewhere else (normally the place where the blood vessels are really thin which is the lungs!) and then they grow again and push through the walls of the blood vessels and into the lung
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