That influx of visitors also guarantees reports of wildlife attacks. Last week, a 9-year-old girl was tossed in the air by a bison at Yellowstone National Park, and over the weekend, a 17-year-old boy was gored by a bison at the Theodore Roosevelt National Park in North Dakota, according to park officials.
"This kind of incident happens every summer, in Yellowstone in particular," Dan Flores, environmental writer and retired professor of the history of the American west, told ABC News.
At Yellowstone, the animals "tend to pay little attention to humans and cars," and will often even come between 5 and 10 feet of cars, Flores said. Bison have injured more people than any other animal at Yellowstone, according to the National Parks Service.
"The tourists who go there tend to think of Yellowstone as a zoo," he added.
Here is how to stay safe from wildlife while visiting national parks:
Remember that the animals are wild
The wildlife at the national parks are not contained "in any way" and come and go within the park as they please, Flores said. Do not treat them like zoo animals.
The first thing that visitors see when entering Yellowstone is a sign of someone being tossed into the air by a bison, Flores said.
"They're not there to amuse you," he added. "This is a very real threat."
Visitors are treating dangerous wildlife that weigh up to a ton "like it's a tame pony," said Mike Mooring, a biology professor at Point Loma Nazarene University in San Diego who spent more than a decade researching bison.
Keep your distance
The National Parks Service recommends that people stay at least 100 yards away from bears and wolves and at least 25 yards away from all animals, including bison and elk.
People tend to get "way, way too close" to the animals, Mooring said, adding that when he was researching bison at a national wildlife refuge in Nebraska, he wasn't even allowed to get out of the truck.
"The fact that people are walking close to these animals and not really having any sense of the danger is really what is stimulating the animals," he said.
In both recent incidents, the victims of the bison attacks were far too close to the animal, Flores said.
"These are wild animals, and if you invade their personal space, that's often how they react," he added.
Pay attention to the animal's cues
People can judge a bison's level of annoyance by two factors: eye contact and their tails, Flores said.
"If you look at their behavior, you can figure out without too much experience if they're riled up and if they're upset about your presence," Mooring said.
Most of the time, a bison will not look at humans at all, so if they turn and look at you and make eye contact, "that's a clear sign that they are becoming annoyed," Flores said. Raising their tails is another sign.
"If you see those two signals, you best retreat quickly," Flores said.
Wild animals are "very individualistic," Flores said, so while one bison may be OK with letting people get close enough to it to take selfies, another one can get annoyed much quicker.
"You can't expect bison to react the same whenever you get too close to them," he said. "Some will be fine -- they'll be chilled out. But others won't be."
Keep in mind that bison can be more reactive during the summertime
The time of year when hordes of people visit the national parks coincides with a period in which bison can be more reactive to human stimuli, according to the experts.
Mating season typically starts in July and runs through September, so the male bison are pumped with testosterone and may become more aggressive and "hyped up," Mooring said.
Young bison in particular are preparing for breeding season, so getting too close to them is like a recipe to get hurt.
"It's similar to getting too close to fighting dogs," Flores said. "Often, they get bitten."
In addition, cows give birth in the spring, so they may be protective of their calves.
"Those you have to be especially careful about because any wild animal protecting young is always sensitive about being approached," Flores said.
Mooring has even been chased by bison cows in the past, he said.
Other wildlife safety tips from the National Park Service
-- Never feed wildlife, as animals that become dependent on humans for food may become aggressive toward people and have to be killed. Also, keep all food, garbage and smelly items packed away when not in use.
-- Never park in the road or block traffic. Use pullout sections to watch wildlife and let other cars pass. Stay in your vehicle in the event of a traffic jam caused by wildlife.
-- In order to protect yourself from bears, be alert, make noises, hike in groups, avoid running and carry bear spray.
-- Be cautious when exiting buildings or approaching blind corners, as cow elk as especially protective around their calves in the spring and hide them near cars or buildings.
-- Wolves are not normally a danger to people, but in the event of an encounter, stand tall and hold your ground. If a wolf approaches you, wave your arms, yell and flares your jacket. If all else fails, throw something at it, or use bear spray.
-- In the event of a mountain lion encounter, always give them a way to escape. Do not run and stay calm. Hold your ground and back away slowly, doing all you can to appear larger, such as facing the lion and standing upright. If the lion behaves aggressively, wave your arms, shout and throw objects at it in order to convince it that you are not prey and "may be dangerous yourself."
--It is best to travel in a close group to avoid black bear encounters. To scare them off, make loud noises, clap your hands or clang pots and pans -- which have all been successful in the past. But, if attacked, fight back, and never try to retrieve something once the bear has it.