Korey Stringer died of heat stroke during the Minnesota Vikings' training camp 12 years ago. Since then, the NFL's summer conditioning ritual has changed a lot, and for the better.

Players have learned there is no courage in challenging the energy-sapping heat and humidity. Coaches have recognized that pushing the guys in uniform — even when they are wearing only shorts and no pads — is foolhardy. Doctors and trainers have discovered every conceivable way to keep everyone hydrated and acclimated to the sweaty conditions.

Yet it is a constant struggle that actually begins long before players report to team facilities in late July, as they are now doing. Once offseason programs begin, and especially during minicamps in June when the temperatures begin to climb, players are reminded to drink up and keep cool.

"Heat illness is 100 percent preventable, and that is the message we try to send to our players," says New York Giants assistant athletic trainer and physical therapist Leigh Weiss. "Each summer prior to our first practice, (senior vice president of medical services and head athletic trainer) Ronnie Barnes presents to the players on the prevention of heat illness and maintaining adequate hydration during training camp. We educate them about the signs and symptoms of heat illness and let them know that dehydration may not only decrease performance and promote injury, but can have very serious consequences."

Few current NFL players were around when Stringer passed away, but many know his story and his legacy. The Korey Stringer Institute was created in 2010 at the Neag School of Education at the University of Connecticut, and it works with the NFL, NCAA and various youth sports organizations to ensure that athletes are well-educated on the dangers of not being heat acclimatized.

Most problems occur at the youth level, particularly at high schools in poorer areas where funding for the most efficient safety measures is not available. Doug Casa, professor of kinesiology at UConn and the lead researcher for the Korey Stringer Institute, fears receiving phone calls about a teenager who died of heat-related problems during a summer football practice. And he gets one or more of those calls nearly every year.

"There are still a scary amount of high schools that don't have emergency action plans," Casa says, noting that just 10 states have met a national guideline for heat acclimatization while 20 more are closing in on the standard.

On the college level, 2003 was a landmark moment because the NCAA became the first major sports organization to mandate heat acclimatization. Casa says the NCAA has been making constant progress in that area ever since.

"That was a huge change and they have had that in place for 10 years and there was only one heat-stroke death in preseason practices. Before that, we were averaging two a year," he said. "So there's a rule change that has basically saved 20 lives."

As for the NFL, while there has been nothing close to a repeat of Stringer's death, it wasn't until the 2011 collective bargaining agreement between the players and owners that significant safety cutbacks in the preseason became the rule.

Under the new CBA, throughout training camp, players can't be on the field for more than four hours per day; only one practice a day can be in pads and is restricted to three hours or less, followed by a three-hour break; and players get one day off per week.

No contact or pads are allowed during the first three days of camp, with the reporting date limited to physical exams, meetings and classroom work. Running and conditioning is allowed.

That is the most vulnerable time for players, especially those coming off injuries. Casa recommends practices of no longer than two hours in the first week of camp.

"I think it helps a tremendous amount, codifying this, and eliminating contact in two-a-days is a very big deal," says Dr. Thom Mayer, the NFL Players Association medical director. "We talk about that from the concussion side, appropriately so, and from the heat-related side.

"Since Korey's death, I think this has improved a tremendous amount. The awareness of club physicians, having a document that is clear for club physicians who must make sure a coaching staff and training staffs are aware of heat-related illnesses. Many (coaches) came up in the era of when you shook it off, you are just a little hot. I think we have made a lot of progress, but that does not mean vigilance has changed. It is a necessity."

That vigilance begins long before players take the field in upstate New York or down in Louisiana or out in Arizona.

"Down here in Houston, hydration is everything because we're practicing in 100-degree heat every day, and I need to make sure my body is ready to go," says 2012 Defensive Player of the Year J.J. Watt of the Texans. "All throughout practice, all throughout the day, I need to make sure I'm hydrating my body and preparing it so it can perform at its peak, and to go about it, it's an all-day thing. It's before practice. It's during practice. It's after practice. It's a full-time job making sure I'm staying hydrated, but it's what keeps me at my best."

Players are encouraged to consume water and Gatorade in meetings, during meals, and in the evening. Players generally weigh in before practice and weigh out afterward, giving trainers an idea of who may be at risk for developing heat illness. Such procedures provide a blueprint for how much fluid players need as a replacement following activity.

Some trainers also ask players to monitor their urine color, which offers a good indication of hydration status. (Dark, apple juice color urine means the player is dehydrated; lighter, lemonade color means the player is hydrated.)

Then there's the cooling-down process. Players are encouraged to remove their helmets in between drills to allow for ventilation. Trainers provide cold towels or sponges during practice, and the ever-popular ice tubs after practice, with a goal of reducing a player's core body temperature.

"You have to be a pro and part of that is to keep yourself hydrated," says Panthers linebacker Luke Kuechly, the 2012 Defensive Rookie of the Year. "It's really up to you, and it's not hard to stay hydrated or to find something to drink. You can have anything you need and they expect you to stay hydrated.

"One of the things I am not sure everyone would know, a lot of hydration comes from the day before and night before. If I do a good job, I don't have to load up and kind of get bloated or heavy."

One issue that doesn't crop up is a player hiding his condition. Casa and Mayer say there's a "buddy system" through which players watch out for each other in the heat.

"By far the biggest indication is an altered mental status, when it does not seem to be the same person in front of you," Casa says. "Some feel excessively hot, feel excessive fatigue, act a little bizarre or different. It's especially true for someone coming off an injury or illness."

So while the idea of trying to make the team at all costs might seem valiant, it is short-sighted and foolish.

The NFLPA's Mayer believes anyone who even gets a look from the NFL should know better, but he recognizes the personality traits that have made them successful.

"All these guys already have been on the right end of the performance spectrum, from junior high to high school to college," Mayer says. "Then, the next thing they know, they are in the league. They always have exceeded expectations, are highly driven and high performance.

"Think about what they do in the offseason and in the weight room, with guys pushing and pushing to have (the most) strength or speed. They are used to training for that.

"And once we start now in camps, they have to make sure not to push it, and to be really careful about the heat. It's incumbent on all of us to be really careful about the heat."