On 15 August 1727 three men and eight boys were left for the day to harvest birds and eggs on Stac An Armin. They were not picked up again for 9 months... 13 May 1728. Surviving on the food they had collected over winter in a cleit.
An ex-St Kildan had died on Harris of smallpox, and as a mark of respect his clothes were sent back. The clothes infected the island, killing 90% of the population... only 4 adults and 26 children were left.
The castaways on Stac Armin who knew nothing of what had occurred were left there to be saved, and finally rescued the following year.
There is confusion over what exactly happened - they may have been on Boreray (a larger and more hospitable rock) and it may have in fact not been smallpox but (equally dangerous at the time) measles.
The '8 Day Sickness' was another catastrophic problem on the island. This was neonatal tetanus caused partly by the tradition of rubbing newborn infants with fulmar oil, and general lack of hygine. The origin of tetanus was not understood until 1884.