The Aquilaria tree grows only in the rainforests of South and Southeast Asia — in Assam, Bhutan, Cambodia, Vietnam, and Papua New Guinea. It produces no scent at all when healthy. The fragrance only comes after infection.
A mold — Phialophora parasitica — wounds the heartwood. The tree responds by flooding the wound with dark, aromatic resin. This resin, built up over decades, is what perfumers call oud. The older the infection, the deeper the oil.