A sudden outbreak of Covid-19 cases on our sister island of Nantucket last week is a sharp reminder of the insidiousness of this virus.

After months with only a handful of new cases, Nantucket health authorities said 38 people tested positive in a week, pushing the Island’s total caseload over 100. The majority of those infected are tradespeople, especially landscapers and cleaners, many of whom not only work but live together, according to the island’s health director, Roberto Santamaria.

Happily, Martha’s Vineyard has not experienced a similar post-Labor Day surge, a point that should be a source of civic pride. In general, Islanders have taken the risk of infection seriously by following health guidelines, including keeping get-togethers small and widespread use of face coverings.

But the Nantucket experience suggests how fragile that record can be. As the weather changes, driving people indoors, the threat of contagion when people gather will only be heightened.

Let’s take the lesson from Nantucket and not repeat it here.