Leeches

The leeches grew massive because they fed on the anabolic steroids in the swimmers' blood. When the team members took steroids during preseason training, their blood contained the growth-enhancing drugs. The leeches picked up at the lake consumed this steroid-laden blood, causing them to swell to terrifying sizes and become increasingly aggressive.

During the lake trip, the leeches were already feeding on the swimmers, setting the stage for their later transformation.

The leeches were carried back to campus after the lake day trip. That night, they were washed down a shower drain where they continued to feed and multiply in the dark campus plumbing, growing larger as they consumed more steroid-laden blood from the swimmers who passed through.

The team members brought the leeches back with them from the lake, unknowingly carrying the threat into their living spaces.

The coach attacked because he had become infected by a leech, which likely implanted a parasitic brood in his body. Just as the survivors were about to execute their electrification plan, the infected coach turned on them, delaying the process and allowing the leeches to close in.

The coach's infection was hinted at by his earlier presence during the leech attacks.

The remaining swimmers and Mariam devised a plan to lure the leeches to the campus swimming pool using one swimmer as bait. Once the creatures entered the pool, they intended to electrify the water to fry them. The plan required precise timing to trap the leeches and execute the electrical surge.

The plan capitalized on the leeches' feeding behavior, using the bait to draw them into a controlled kill zone.

In the film's final moments, one of the surviving swim team members is revealed to have been the mad scientist behind the leech project. The twist shows he had a hand in creating the monsters and confirms that breeding stock still exists, suggesting the threat is not truly over.

The ending revelation reframes earlier events, hinting that someone on the team had knowledge of the leeches from the start.

The swim team took anabolic steroids during preseason training to boost their athletic performance. The practice was conducted with some secrecy, and it triggered an escalating rivalry while giving the team a darker edge, all of which directly contributed to the leech catastrophe when the parasites fed on the drug-enhanced blood.

The steroid use was introduced early as a seemingly normal athletic practice that would have deadly consequences.

The ending confirms that breeding stock still exists, hinting that the surviving scientist has maintained a population of the engineered leeches. This open-ended conclusion leaves the threat unresolved and suggests future danger, giving the film an unsettling, incomplete victory despite the immediate massacre being prevented.

The mad scientist's reveal implies he had been cultivating the leeches long before the lake incident.

The plan was delayed when the leech-infected coach attacked the group just before they could throw the switch. This critical delay allowed the leeches to advance closer, resulting in another swimmer being killed before the group finally subdued the coach and successfully electrified the pool.

The coach's infection had been building throughout the crisis, culminating in his violent interruption at the most crucial moment.

Mariam was the girlfriend of one of the swimmers and joined the surviving team members in devising and executing the plan to eliminate the leech threat. She played an active role in the group's strategy to lure and kill the monsters using the bait and electrification method.

Mariam's presence as a non-team member gave the group an outside perspective in formulating their survival strategy.

The leeches targeted the swimmers because their blood contained anabolic steroids, which served as a powerful growth stimulant for the parasites. Once the leeches had fed on this enhanced blood, they became dependent on it, causing them to hunt the very team members who had originally carried them.

The leeches' attraction to the swimmers was established at the lake when they first fed on the team members.