The water-borne infections that struck guests of the Playboy Mansion are more common than you think.

According to the Los Angeles County Health Department, about 200 people who attended a recent Playboy Mansion fundraiser were infected by Legionella bacteria. Four of them came down with Legionnaires’ disease, a sometimes deadly pneumonia; the rest had Pontiac fever, a flu-like illness that lasts about three days.

As many as 18,000 people in the United States contract Legionnaires’ disease each year, according to the Centers for Disease Control and Prevention (CDC), 5% to 30% of whom die from it within a few days or weeks.

About 90% of Legionnaires’ cases go undetected, in part because physicians treat the pneumonia but don’t test for Legionella bacteria.

“That’s a big problem,” says Matt Freije, author of Protect Yourself from Legionnaires’ Disease: The waterborne illness that continues to kill and harm. “If Legionella is not recognized as the cause of the infection, then no investigation is performed to pinpoint and fix the plumbing system, hot tub, or other water system that caused it, and that water system can continue to make people sick.”

Pontiac fever is even less likely than Legionnaires’ disease to be diagnosed because it is less severe. Most of us don’t even go to the doctor when we get the flu, let alone try to find out what caused it. Generally, Pontiac fever is recognized only when physicians in the same vicinity report several flu-like illnesses at the same time, attracting the attention of the health department, as with the Playboy event.

According to Freije. “Laws in much of Australia and Europe require building owners to maintain water systems to minimize Legionella bacteria but prevention is not mandated in the United States. I have received emails from many Legionnaires’ survivors who were outraged to find out they suffered from a disease that is preventable.”

Steve Sederstrom is one of the Legionnaires’ survivors who tells his story in Freije’s book. “My experience with Legionnaires’ disease was the worst thing I have ever been through. After five months I still have a major problem with short-term memory-I forget where I am going, or forget people’s names even though I have known them for years. I am afraid that I will never get my memory back.”