Taking a dip in the pool on holiday to cool off from the sun is always a treat.
But as inviting as a swimming pool may appear, it could actually be hiding a number of deadly bacteria which could be harmful to your health.
Safe to swim?
Even if a swimming pool abroad looks completely clean, it may still be harbouring some harmful parasites - most commonly Cryptosporidium and Giardia, both of which cannot be seen by the naked eye.
Cryptosporidium can be spread when water has been contaminated and a pool is not properly cleaned.
It is one of the most common causes of stomach infections and can lead to cryptosporidiosis, causing symptoms including diarrhoea, nausea, fever and stomach pain.
Symptoms typically last around five to six weeks, according to the NHS, although this can sometimes be longer if you have a weak immune system.
The infection does not usually require treatment, with individuals simply advised to drink plenty of fluids to re-hydrate.
Giardia is a highly infectious stomach bug which can be caught from water getting in your mouth while swimming in pools.
Symptoms can include smelly diarrhoea, stomach pain or cramps, flatulence and bloating.
The bug is treated with antibiotics and symptoms should stop in about a week, although you may have to have a stool sample tested before treatment.
How to stay safe
If you are concerned about the cleanliness of a swimming pool abroad, it is advised you check the clarity of the water before taking a dip, Luke Griffiths, qualification development manager at Safety Training Awards advised.
He told The Sun, "Be mindful that cloudy looking water can indicate problems with the pool and the water may contain harmful micro-organisms or dangerous chemical properties."
Griffiths also warned holidaymakers to stay out of the pool if the clarity of the water is "so poor you can't see the bottom", or if there is evidence of a "slimy coloured growth on pool walls, floors and tiling", as this could be a sign of algae or other biofilm growth.
"This could harbour unhealthy micro-organisms, such as Pseudomonas aeruginosa,” he said.
To check whether a hotel is considered safe, holidaymakers can visit Check Safety First before travelling which lists all of the resorts around the world which have passed their inspections.
Alternatively, guests can ask their hotel to perform a chemical and microbiological test of the swimming pool water to check for bacteria.