During the swine flu pandemic in 2009, they helped run clinics and designed an online flu-shot scheduling system now used across the state. After last year’s tornadoes in Western Massachusetts, they managed a shelter for people whose homes were damaged. At the Boston Marathon’s finish line each April, they guide injured runners to medical help.

The busy crews belong to the Medical Reserve Corps of Region 4A, formed almost eight years ago as part of a national system of local networks ready to respond to public health emergencies, created in the wake of the 9/11 terrorist attacks. But this largely volunteer group — the largest in the state, with about 4,800 participants representing 32 communities roughly bounded by interstates 95 and 495 — was deployed 10 times last year across Massachusetts.