This would apper to have been the first smallpox epidemic to hit the Island - smallpox had become endemic in London by around 1680 and in the North of England by 1684. There is a note against the burial of Jany Cluage in Braddan on 17th Jan 1685 - "first that died in small pox". From this year smallpox was a major factor in the death of children.
The plot of burials/week for various parishes shows the spread:
The extant registers cover the major towns, though Lezayre, which would cover part of Ramsey, does not start until 1690 thus the Maughold burials may not show the complete picture. The rural parish of Jurby also shows a single sharp peak of 5 burials, Ballaugh and Andreas burial registers are deficient for this period; the registers for other parishes do not start until after 1690. (Braddan, German and Michael are from personal transcriptions, others are from indices).
Easter Sunday fell on the 19th April 1685 which may have facilitated the rapid transmission due to the Easter Church services in Malew and German.
Smallpox has an incubation period of some 10-12 days before symptoms are seen, after which the infected person can spread the disease by close contact thus several members of the same family are likely to be infected, with transmission faster within towns. Note the particularly rapid onset of the disease within the major towns. The mortality rate is some 25 to 30% of infected persons - it is particulary dangerous to children (less so to infants) - the following chart shows German burials (children are those either indicated as child of or son/daughter of).
Similar rapid onset can also be seen in the 1764/5 epidemic.