Pulmonary hypertension can happen without any known cause, which is rather frustrating for those who are diagnosed this way. This is known as idiopathic pulmonary hypertension.
Pulmonary hypertension can be diagnosed secondary to a cause, like connective tissue diseases, congenital heart defects, chronic liver disease, HIV, drugs, and toxins. This is known as associated pulmonary hypertension.
Pulmonary hypertension can also be genetic, which means it can be inherited. This is known as heritable pulmonary hypertension.