This is one of the best episodes in the series benefiting a great deal from an excellent central performance by Peter McEnery as a paranoid hospital attendant. His obsession with the number 9 and its association with evil gives this almost the feel of a black comedy as he tries to persuade others to his 'logical' way of thinking (especially the local parish priest and the young woman who lodges with him and his mother, and who harbors an affection for McEnery despite having a kid in tow).
There's even a surreal dream sequence in which the girl assumes a beastly countenance and most other characters (who are shown to be in on the conspiracy) invite the hero to eat the woman's baby a perverse Satanic twist on the Eucharist! as the only way he can exorcise the demon within him
but the priest, wielding a large cross, comes to his aid. Unfortunately, however, the abrupt denouement to it all is rather limp!
Incidentally, this also makes prescient comments about the danger of transmitted diseases which would soon become the topic of the day with the advent of AIDS, not to mention the omnipresent threat of biological warfare! By the way, given the similar title of an entry in the following Hammer House series called MARK OF THE DEVIL (1984) which I watched back in the day when it aired on local TV! I was mistaken into thinking it was a remake of this one; the "mark" is very different: it has a psychological effect here, whereas it takes a physical shape in the later episode.