It was Paul's eyesight. Paul usually had a scribe to pen his epistles, but sometimes he would write some short portions in letters large enough that he could read what he was writing because of his poor eyesight. Demons that inhabit the body usually manifest themselves in physical infirmities. Most of these demons can readily be cast out unless, as in Paul's case, the demon is sent by the Lord to buffet that person to keep him from being proud or for some other reason. Demons can be sent in this way if a Christian needs chastening by the Lord and that Christian does not respond properly to less severe measures by Him.
Demons can also inhabit the realm of the soul.
In these cases there is usually no physical manifestation, but rather deviant or sinful behavior. This would include consulting horoscopes, Ouija boards, fortune tellers, and so forth. Also, voluntarily listening to many types of rock music available today and reading New Age and other occult books. Pornographic literature would include many of the seemingly harmless department store catalogs that Christians bring into their homes. One of the greatest needs in conservative churches today is scriptural teaching about spiritual warfare.