That's also the reason, why a lot of substances for medicine are produced by fungi. One of the first example and probably the most famous one is penicillin, which was used during the Second World War. The word goes, that it even helped the Allies to win the Second World War, and without it would be a different history. The advantage to have something against bacteria on the battlefield is crucial. But also in Traditional Chinese Medicine fungi are used a lot. And also today research for medicine to fight cancer is funded by the National Health Organization of America.
The Difference between the Body and Fruit Body
Mycelium is the body of fungi. These 'roots' are made out of hypha. This body is the core of the fungi, and the mushroom itself is just the fruiting body of fungi. To give you an example: if you think about an apple tree, the whole tree is the body and the apple is the fruiting body. A mushroom is really just there for reproductive purposes. Here the spores get produced and here you can find the gender.
Another example is moldy bread. The moment you notice it, the bread is probably already infested with the mycelium.