DEFINED by its Latin rhythms and party atmosphere, this one's irresistible. In the five years since it all started, more than 3.5 million Zumba DVDs have been sold through infomercials and about 20,000 instructors in 40 countries now teach it, according to Alberto Perlman, the chief executive of Zumba Fitness, in Hollywood, Fla. The man who started it all, Alberto Perez, 37, told the New York Times he still does not fully grasp the empire he has created. “Remember when Forrest Gump started running and people followed him?” Perez said. “I feel like the Forrest Gump of dance.” Perez, a former aerobics trainer from Colombia, said he stumbled upon his fitness concept by accident. In 1986, as he was about to teach a class in Cali, Colombia, he realized he had forgotten his aerobics music tapes. He grabbed his own music from his car — salsa and merengue from El Gran Combo, Las Chicas del Can and other popular bands — and improvised the class. From then on, he said, his students refused to go back to the old tum-tum-tum-tum aerobics beat. He taught in Bogot?, Colombia, and, in 1999, in Miami, where he got together with Perlman and another fellow Colombian, Alberto Aghion, to offer Zumba videos through infomercials. A session of Zumba, a cardio-dance routine, can feel like a trip to the nightclub of a cruise ship, where a well-toned crew member teaches you to wiggle your hips and do the fast footwork for a mix of dance styles to the thump of loud music. People often throw away inhibitions — they pump their arms, applaud, let out “yeahs” — as they work out. Over the course of an hour, a Zumba class will span a variety of dance rhythms, like mambo, cha-cha, cumbia, and merengue, with the occasional hip-hop or belly dancing move thrown in. Recent Zumba DVD additions include ZumbAtomic, a program for children 5 to 12 ; Zumba Gold, classes for people 60 and older; and Zumba Toning, a sculpturing program using weighted sticks that sound like maracas, offered on DVD and through classes starting this year. Still to come are water exercise classes called Aquazumba, and a guide to all things Zumba (including a nutrition plan) to be published next spring. Zumba's appeal is that it requires no licensing fees or investment in materials, just a Zumba-trained teacher. (The training program is not rigorous: an eight-hour workshop is all it takes to become an instructor, with continued learning through CDs and DVDs and online access to new steps, music recommendations and tips.