A summer trip to Shenandoah National Park usually means fresh air and natural beauty — but recently some visitors have encountered something far less welcome: a highly contagious illness spreading along the trail.

The National Park Service says there have been recent reports of severe sickness along the 101-mile Appalachian Trail, and the suspected cause is norovirus.

Because the virus spreads easily, the Park Service is urging visitors and hikers to take precautions to protect themselves and others.

First, treat all water, since most water filters do not remove viruses. The Park Service says boiling water for at least one minute is the most reliable way to kill germs.

Remember to wash your hands with soap and water for at least 20 seconds. The NPS says hand sanitizer doesn’t work very well against norovirus.

Don’t share food, water or other personal items, since the highly contagious norovirus can be spread on surfaces.

To properly dispose of waste, according to the Park Service, “bury poop at least 8 inches deep and 200 feet from water sources, campsites and trails. Wipe down privy handles, bear boxes and other touch spots with bleach wipes before using.”

If you have symptoms, try to avoid contact with others, including tenting alone if possible and getting single rooms at hostels and hotels.

To report cases of norovirus, email publichealthprograms@nps.gov.