The England midfielder said he had not tried to hide the fact he had asthma - it became apparent when he was seen using an inhaler after playing for Los Angeles Galaxy in the MLS Cup final in Seattle.
Beckham said: "It's out there now. Sometimes I have good days and bad days. I've never hidden it but it's something I've had for a good few years now.
"I hope it turns into a positive because I've been able to play for many years with the condition. I know there are many other players who have overcome it such as Paul Scholes."
The 34-year-old also dismissed a report that he smokes cigars.
"I've always prided myself on the way I look after myself," he said. "On special occasions, like winning the European Cup, I might have had a puff. But I don't sneak into the garden - I think my boys would kill me if I did."