Something occurred,probably long ago. AMy have been a severed roots or womething else.

In any case, part of the tree died. The fungus supporting those mushrooms mushrooms is decomposing the dead wood.

If you need advice re its stability, I suggest you have a Certified Arborist or two make an on-site visit to evaluate it.

Is there anything I should do now? Would an anti-fungal help? Anything I can do to prevent the tree from continuing to die?

Fungal growths like that on the outside of the tree, specially on major branches or the trunk, are typically an indication of heartwood rot and that is incurable.

Your tree is on the way out. You can be proactive and remove it now before other large portions die off or you can let it linger. But it IS dying and irretrievably so.

Thank you guys very much for your help. One followup question. If I'd like to have the same tree here could I start growing one next to it and eventually remove the "infected one" Also would the soil or the proximity affect the new one? is it "contagious?" Anything I can do to prevent this from happening again?

Thank you very much
~Luke