PING! The illuminated seatbelt sign above your head flicks off and you start to relax and settle in for the long flight ahead. Shoes off, complimentary travel socks on, seat reclined to its full extent without a thought for the person sitting behind andyou are ready to get stuck into the first whisky and soda the stewardess is handing to you. Aaaachoo! You spill the whisky, by now fortified by a foreign body or two, as your neighbour sneezes over you. Sniff, snort, cough, splutter and copious amounts of throat clearing later - and you realise with a sinking feeling the person occupying the next seat for the 15-hour flight is in less than perfect health. What can you do about it? The answer is very little. If you are a pessimist, you might as well buy shares in medicated lemon teas or extra-strong menthol sweets and book two weeks' sick leave. And according to Hongkong medical experts, there is nothing to be gained by demanding to sit next to someone who is a picture of health. The runny-nosed co-passenger could be sitting rows away from you, but it will not stop the germs coming your way. Doctors say air passengers are sitting ducks for airborne infections, and short of strapping on a special air-filtering mask for the duration of the flight, there is little you can do to avoid breathing potentially contaminated air. Germs do not respect the width or your wallet or capabilities of your credit card. It makes no difference whether you have paid top whack for first class or have been herded into the cattle class section with all the other economy passengers - you all breathe the same air. The common cold can be the least of your worries. The risk of infection while flying was highlighted last week when Hongkong's flagship carrier Cathay Pacific issued an alert to more than 650 passengers after a Canadian traveller was diagnosed as having tuberculosis. A respiratory illness, TB can be fatal if not treated with antibiotics. Initially affecting the lungs, the disease can also damage the liver and kidneys. Hongkong-based Dr John Simon said trying to avoid infection on a plane was ''craziness'', and said passengers who were ill had a responsibility not to fly. ''People should be advised not to travel if they have flu. There's nothing else you can do.'' He said there was inevitably an increased risk of infection through recirculating air in a plane, and said airborne diseases included, ''colds, flu and TB''. Not all doctors are so convinced that flying can damage your health. One Hongkong family doctor said the risk of plane-related infection was ''not very high''. But she said a passenger infected with rubella or chicken pox was likely to infect some others on the flight, while colds and flu might affect people nearby. She said for those truly paranoid about infection, the only answer would be a face-mask similar to those worn by paint-sprayers: ''They have very high filtering capabilities, and wearing one almost nullifies the possibility of catching anything.'' A Cathay spokesman said a 747 held 1,120 cubic metres of air. Every hour there were 10 to 12 changes of cabin air, half of which was fresh and half recycled air. ''We have filters which absorb 99 per cent of dust and bacteria, so the recycled air is clean and fresh,'' he said. Cathay contacted its passengers at the request of the Canadian health authorities after they discovered the Canadian man was suffering from TB. But a chest specialist said that while some of the passengers, from Hongkong, Thailand, Canada and the Philippines, may test positive for TB, they would not necessarily have contracted the disease from the infected passenger, as half the population of Southeast Asia was thought to have been in contact with TB.