Mould will form anywhere you have moisture for a long period of time at a warm temperature. Some moulds grow and feed on organic materials including drywall paper, wall paper glue and of course wood. Other moulds simply grow on the surface, which means that moulds can grow wherever they can find moisture and warmth. Many don't even need light to grow, so they can easily grow inside the walls.
Recent research by the CMHC, Canada Mortgage and Housing Corporation, has shown that many types of moulds found inside Canadian houses can irritate breathing, some can cause asthma and others are simply fatal. In general, you should not live with mould.
In most houses you can clean up and control mould yourself by keeping humidity down, stopping condensation on cold surfaces and cleaning up any signs of mould quickly. There are various professional services available as well, from air quality testers to mould abatement contractors (usually the guys that were trained to clean up asbestos).
CMHC and Health Canada are the two best resources for information about mould, with many publications about mould itself, about cleaning up after a floood, about avoiding mould.