From an interview with Dr. Yeshi Dhonden, author of Health through Balance.
Dr. Dhonden, how does a spirit make a person sick?
How that happens, generally speaking, is you have various types of spirits, could also be nagas – the same kind of thing – and the spirit harms you with their mind. The spirit directs malevolent thoughts to the body, and that’s enough to create illness.
Nagas dwell in various places like springs, also sometimes in groves of forests and other places. Especially in the case of forest glens or in springs, if someone comes and pollutes the spring or starts cutting down trees that are actually the lodgings of nagas, they can harm you. But in such a case, it’s really up to a lama – and I am not implying that I’m a lama in this particular case – then it’s up to a lama to perform a divination to really find out what kind of spirit influence it is, to identify that. It’s primarily up to the lama to determine what type of ritual practices, etc. need to be done to counteract that influence.
So if it’s a lama who is diagnosing or trying to determine what type of spirit influence is afflicting the individual, that’s how the lama will do a divination. If it’s a doctor, the way the doctor proceeds is to do a special kind of diagnosis of the urine, unlike the normal one for normal illnesses involving imbalance of the various humors. By analyzing the urine, the doctor will be able to tell what type of spirit is involved, and that would indicate what kind of countermeasures need to be taken.