Story highlights Four people have been attacked by Yellowstone bison this year

Visitors are asked to stay at least 25 yards away from large animals in the park

(CNN) Bison attacked two more visitors at Yellowstone National Park over the past week, for a total of four attacks in the park so far this season.

A 68-year-old Georgia woman remained hospitalized Thursday after encountering a bison while hiking Wednesday on Storm Point Trail, according to the National Park Service.

As she passed the bison, it charged and gored her. A witness reported the attack to a nearby ranger leading a hike, who called for help. Due to the serious nature of the woman's injuries, she was taken by helicopter ambulance to a hospital outside the park. Her condition was unknown Thursday.

The other incident happened on June 23, when a 19-year-old Georgia woman and three friends who work at nearby Canyon Village were walking to their car after a late-night swim at the Firehole River. They saw a bison lying about 10 feet away, and one friend turned and ran. But the animal charged the teen and "tossed her in the air," the park service said.

After the teen went to bed, she awoke a short time later feeling ill and called for medical help. She took an ambulance to a hospital outside the park and "was released with minor injuries later that day," the park service said.