From the Cape Cod National Seashore:
Mosquitoes in and around the Cape Cod National Seashore area have tested positive for both Eastern Equine Encephalitis (EEE) and West Nile Virus. These mosquitoes gravitate towards mammals and are most active between dusk and dawn. Avoid outdoor activity during these times. If you must go out, wear long-sleeve clothing that covers as much skin as possible. Apply FDA approved insect repellent (such as DEET) to any exposed areas.
The state of Massachusetts has information about about choosing and using repellents here: https://www.mass.gov/service-details/mosquito-repellents