What will happen if yellow fever goes untreated?

There is no specific: treatment for Yellow Fever other than preexposure vaccination against the virus and avoiding mosquito bites in endemic areas (Africa). Treatment is really just for the complications associated with the illness such as analgesic and antipyretics. If liver failure and/or bleeding develops, that would require supportive management.

Yellow Fever: There are no specific treatment for Yellow fever. Rest, fluids, pain relievers & fever reduction meds. Prevention is best course of action which may include yellow fever vaccine.