The last thing you want after a week away is to spend a couple of hours on a flight filled with people coughing. All that hard work you spent relaxing in the sun could be undone by catching a cold from a fellow passenger.

But can flying really increase your chances of becoming sick? Sally Bloomfield, honorary Professor at London School of Hygiene and Tropical Medicine, says being in a confined space – much like other forms of public transport – can mean you’re more likely to become unwell. “Planes are just like any other living environment,” she tells HuffPost UK. “The more crowded it gets, the more likely there is to be a spread of harmful germs.”

It doesn’t matter how often or thoroughly a plane is cleaned because the moment people board that plane, they are shedding millions of microbes – including some harmful germs. “It’s not the planes that are dirty, it’s us – we are shedding microbes all the time,” she adds.

A 2018 study found passengers on flights had an increased chance of becoming ill if they sat in the wrong place – people passing by, or sitting in the row directly in front of or behind an ill person, were most likely to become sick. The illness was often transferred via the eyes, nose or mouth, scientists noted. But unlike other forms of public transport, it’s not so easy to get up and move once you’re on a plane.

Prof Bloomfield, who is also chairman of the International Forum on Home Hygiene, says there are two ways of becoming infected when travelling on flights – and, indeed, other modes of public transport. “One is through the air and the other is through touching things that are infected with our hands, and then touching the mouth, eyes or nose,” she explains.

Contrary to what you may believe, there is no “good evidence” that shows air conditioning spreads organisms. “If there’s one thing that will spread through air conditioning it’s viruses because they are tiny organisms, so if you try to filter them out, you won’t be able to,” Prof Bloomfield adds.

So how can you stop catching an illness on a plane? Prof Bloomfield has three rules she swears by when flying.

The first is hand hygiene: either washing hands regularly or (the easier option on flights where movement is restricted) carrying hand sanitiser and using it often, especially before eating.

“If there’s anything that’s going around it’s going to be spread by your hands,” she says.

Cold viruses can be transmitted in the air, but also on surfaces, which leads to the next point: try not to touch your eyes, nose and mouth, as these areas are prime gateways for germs to enter the body.

Lastly, when she’s flying long-haul Prof Bloomfield will often use a nasal spray designed for treating the common cold. She adds: “Dose yourself once you get to the other end because it takes time for the cold virus to burrow into the tissues of your nose, and if you catch it in the early stages, you can prevent a cold.”