Classes and fitness rooms are set to be busier - and sweatier - than usual, as thousands of gyms and clubs are opening their doors for UK National Fitness Day. Meanwhile, 10 million Britons are already members of a gym.

It is usually up to members to use paper towels and disinfectant spray to clean off equipment another person has used - otherwise bacteria can lurk for several hours and viruses for several days.

"If you swab a yoga mat you probably are going to pick up viruses and certainly funguses," says Dr Seth Rankin, a GP and chief executive of the London Doctors Clinic.

"Minor things like athlete's foot can be picked up in any moist environment, it's why we wear flip flops in the gym showers," he says.

'Herpes' mats

A US surgeon - Dr David Anthony Greuner - recently issued a more serious warning by claiming that herpes, a virus more commonly associated as coming from sexual contact, could potentially be picked up from dirty mats.

He says in a blog post: "Making skin contact with a dirty yoga mat covered in germs and bacteria can lead to skin infections, acne, toenail fungus and even transfer of the herpes virus and staph and strep infections in susceptible individuals."

Meanwhile, a study in Sexual Health, a medical journal, also found a low risk of getting human papilloma virus (HPV), an infection which can lead to genital warts, by using unclean bike seats at the gym.

But GP Dr Rankin insists the risk of catching a more serious infection from a mat or seat is "vanishingly rare" and tells people not to worry.

"If you were doing yoga on a naturist farm you'd certainly want to be very careful," he says, since herpes is usually spread through mucous membranes or broken skin.

"But where people are wearing Lycra or normal gym clothes the chances are incredibly slim," he adds.

But can we rely on gyms themselves to keep the equipment clean?

Sue Millward, Nuffield Health's lead for infection prevention, who monitors hygiene in its 111 gyms, says staff wipe mats and exercise equipment at the beginning and end of each day.

But she says it is impractical for gyms to clean machines after every workout.

"We keep an eye on what's going on and who's using equipment," she says. "If our staff see that somebody has been perspiring a lot and not cleaning they may go round and clean it."

Busier gyms do get cleaned more often, but Ms Millward says Nuffield also puts up signs to encourage members to wipe down equipment once they are finished.

She adds: "If somebody is unwell, they're not generally going to be at the gym and infecting others, and we do tell people not to come in if they've got illness or flu."

Smelly kit

The best way to avoid getting ill from the gym is to accept that you will be picking up germs during a workout - and wash your hands and kit afterwards, says hygiene expert Dr Lisa Ackerley.

"Don't get too hung up about it," says Dr Ackerley, a chartered environmental health practitioner.

"There's a risk of cross-infection where you share equipment, but people do it and we're not dropping dead from it."

She says people usually have a shower after a workout - but during exercise, should avoid rubbing their eyes or touching the top of a drinking bottle.

She adds: "Kit can get really smelly, even if it's being cleaned, because people wash it at a low temperature."

She says a sure-fire way to kill germs is to wash clothes at 60 degrees Celsius, or consider using a laundry sanitiser.

She adds: "It's simple really, after you've been on your gym equipment, don't go and have a sandwich without washing your hands."