If untreated, pulmonary embolism can be fatal. In fact, pulmonary embolism is fatal for around one-third of undiagnosed cases. However, if a doctor successfully diagnoses pulmonary embolism, prompt treatment can increase someone's chance of survival up to 95 percent. In undiagnosed cases, the likelihood of death is high within the first couple hours after the pulmonary embolism arises. Some people with undiagnosed pulmonary embolism experience unexplained pulmonary hypertension, in which sections of the heart wear out from the immense strain it takes to pump blood through both clogged arteries and the remaining open ones. People most at risk for the fatal complications of pulmonary embolism are those who already have a serious heart or lung disease. However, most healthy people can survive minor pulmonary embolism with medical help.