(USA TODAY) -- Bisons have attacked four people at Yellowstone National Park so far this season, prompting the park to issue a reminder that visitors are responsible for their safety and should stay at least 25 yards away from wildlife.

In separate incidents, four people were injured after getting too close to bisons, the park said in a statement. In the first two incidents, visitors to the Old Faithful area in the park were injured and flown by helicopter ambulance to hospitals.

On June 23, an off-duty concession employee came upon a bison while walking off the trail after dark in the Lower Geyser Basin area. And on Wednesday, a visitor encountered a bison while hiking the Storm Point trail in the Yellowstone Lake area.

Four incidents in less than two months is a lot more than usual, Yellowstone spokeswoman Amy Bartlett told CNN. "We usually have one to two incidents per year," she said.

Nearly 5,000 bison live in Yellowstone, located mostly in Wyoming. It's the only place in the United States where the animals have lived continuously since prehistoric times, according to CNN. With the July 4 weekend, the nation's national parks are entering their busiest season, the network reports.

"Visitors should remember that while many of the bison and elk in the park may appear tame, they are wild animals and should never be approached," Yellowstone National Park said in a statement. "Bison can sprint three times faster than humans can run and are unpredictable and dangerous."

Park officials added that visitors should stay at least 25 yards away from all large animals including bison, elk, bighorn sheep, deer, moose and coyotes, and at least 100 yards away from bears and wolves.

"If a visitor comes upon a bison or elk along a trail, boardwalk, parking lot or in developed areas, visitors must give the animal at least 25 yards by either safely going around the animal or turning around, altering their plans, and not approaching the animal," Yellowstone National Park said.

In the most recent incident Wednesday, a 68-year-old woman from Georgia was hiking on the Storm Point trail and encountered a bison nearby. The woman continued on the trail and as she passed the bison, it charged and gored her. A witness ran up the trail to report the incident. The woman was transported by helicopter ambulance to a hospital due to serious injuries.

On June 23, a 19-year-old woman from Georgia and three friends were returning to their car after a late-night swim when they spotted a bison lying down about 10 feet from them. The three friends turned and ran from the bison. But before the woman could react, the bison charged her and tossed her in the air. The woman later went to a hospital and was released with minor injuries.

On June 2, a 62-year-old Australian man visiting Yellowstone was seriously injured after reportedly getting within 5 feet of the bison while taking pictures, CNN reported. The network also notes that on May 15, a 16-year-old Taiwanese girl was gored by a bison while posing for a photo near Old Faithful, Yellowstone's famous geyser.