“I’m not a biologist but I know that evil spirits don’t cause disease.”
There are Christians, Muslims, Buddhists, Hindus, atheists, and many others who believe evil spirits exist and can cause illnesses. Being a biologist or even a medical doctor does not preclude one from such a belief.
There are many Christian missionaries who are medical doctors and have extensive, first-hand experience with both natural illnesses and those with spiritual causes.
In the Bible, Jesus healed those who were sick from natural maladies as well as those afflicted with evil spirits. Some of these had physical manifestations which mimicked natural illnesses such as being crippled, deaf or mute.
The existence of these things does not depend on anyone’s belief in them.
I knew several priests experienced with exorcisms who absolutely believed in demonic possession and told very convincing stories as to why.