Millions of contact lens wearers are at risk from an eye-devouring amoeba that can cause blindness, experts have warned.
Acanthamoeba is abundant in tap water as well as rivers, ponds and lakes.
It can infect anyone who fails to clean their lenses properly, or keeps them in a dirty case, yet few people are aware of the parasite, and the condition it causes is often diagnosed wrongly or missed.
Each year in the UK doctors treat around 75 people infected with acanthamoeba.
The number is small, but anyone unlucky enough to pick up the organism is in for an unpleasant experience.
Acanthamoeba sticks to contact lenses and burrows its way through the cornea, causing acute pain. It feeds on bacteria and the corneal tissue.
Treatment requires hospital admission and round-the-clock administration of disinfecting eye drops.
If this fails to work, a corneal transplant may be needed, but in the most serious cases, the organism is so deeply embedded it cannot be removed and blindness results.
An estimated 3.7 million contact lens wearers in the UK and more than 125 million worldwide are at risk from acanthamoeba, say scientists.
Dr Fiona Henriquez, from the University of the West of Scotland, said: "It is a potential problem for every single contact lens wearer. The incidence is quite low but that may be a problem with diagnosis."
She added: "There are no effective drug treatments. The drugs used are often ineffective and it's a very brutal regime.
"It requires hospitalisation and topical applications of a toxic substance to the eye.
"We're trying to improve the elimination of this parasite and prevent blindness."
Graeme Stevenson, an optician working with the scientists to develop better contact lens solutions, said: "A lot of infections are caused by rinsing contact lenses out with tap water or swimming or showering while wearing lenses."
It could take just a week for the organism to penetrate deep into the cornea and cause scarring, leading to cloudy vision. Infection is much more likely if there is an abrasion on the cornea.
Single use daily contact lenses are the safest option, and lens cases should be changed every week, said the experts, who issued their warning at the British Science Festival at the University of Aberdeen.
The Canary Islands is an acanthamoeba hot spot because of the way water is stored there, they added.
Holidaymakers are especially at risk because they tend to be less careful about contact lens hygiene.