"Sometimes the body has a natural reaction that is not helpful," Epstein said of this phenomenon. "The fluid comes out of your blood stream and invades the lung, and then we have a lot of fluid inside the lung, leaving no room for the air.
"Then you have pulmonary edema," said Epstein.
Pulmonary edema, or the accumulation of fluid in the lungs, Epstein explained, can later result in cardiac arrest, as oxygen is prevented from getting into the blood stream and eventually stops the heart from beating.
Johnny's death may have also been a result of the chlorine in the pool water, Epstein said.
"The concentration of the water [Johnny swallowed] could have caused a lot of inflammation in the lung," Epstein said. "And then, the body's reaction to inflammation is to send in all sorts of fluids to fight it -- and with that, your lungs are filled with fluid."
Water safety experts advise parents to keep a constant eye on their playing children, and be aware of complaints of difficulty breathing.
"People -- especially children -- need to be supervised around the water with vigilance, even if there are lifeguards present," said Gerald Dworkin, a water safety expert, who has developed safety training programs since 1984 for Life Saving Resources. "Anyone who has been submerged and has aspirated should seek medical attention."
The Centers for Disease Control estimates there were 3,582 fatal unintentional drownings in the United States in 2005, the most recent year for which data is available, and more than one in four drownings are children, 14 and younger. The CDC does not keep statistics on the number of secondary drownings.
Jackson says she is certain there is nothing she could have done to prevent her son's death, but hopes that, by sharing her story, other parents looking ahead to a summer filled with swimming, will be more cautious.
"If your child comes out of the pool and seems sleepy or lethargic, watch them very, very closely," Jackson said. "Rush them to the hospital or call 9-1-1 immediately."
"It's better to be safe than sorry."