A microbiologist has shared a simple ritual he does every time he comes back from holiday.
Dr Tom Makin leaves his shower running for 20 minutes without getting into it, and after it's turned off he doesn't go back in the bathroom for another 20 minutes.
The process, he says, is a measure to prevent lethal bacteria causing life-threatening conditions.
Dr Makin says that in the time a person is on holiday, stagnant water in your shower head could turn toxic.
This means people are at greater risk of inhaling bacteria called legionella.
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Legionella can cause pneumonia and organ failure, both of which can be fatal.
He told the Mail on Sunday : "I cover my nose and mouth with one hand and turn on the shower with the other," he explains of his first task when he gets home after a break.
"After 20 minutes or so, I’ll turn the shower off and then I won’t go back into the bathroom for at least another 20 minutes."
And going away in the summer increases the risk, because legionella thrives when the temperature is between 20C and 45C.
Garden hoses and sprinkler systems are also risky.
The World Health Organisation says that legionella is on the rise, in likelihood because people are taking showers rather than baths.
An estimated 500 Brits contract the condition each year.
