AIDS sufferers - and other victims of certain rare conditions - are being alerted following the outbreak of an infectious disease.
Health chiefs say they believe the disease - cryptosporidiosis - has peaked locally.
The normal symptom is mild diarrhoea which can last for several days or longer.
Around 10 cases have been discovered in Chorley and South Ribble.
It can be particularly serious for people suffering from AIDS or other conditions that lower the body's immune system.
The worst-hit health authority areas are South Lancashire, North West Lancashire, Salford and Trafford and Wigan and Bolton.
In the past, outbreaks have been associated with water consumption, animal contact, swimming pools, or person-to-person spread.
The current outbreak was discovered when a water sample from Thirlmere Reservoir in South Cumbria was found to contain unusually high levels of cryptosporidium three weeks ago. However subsequent samples have been normal.
There is no effective treatment for cryptsporidiosis and it is said to clear up naturally in one to four weeks.
People with symptoms should ensure they drink plenty of fluids and take care with personal hygiene.
Though there is said to be no need to boil water, AIDS/HIV sufferers, as always, should.
Dr Kenneth Lamden, consultant in communicable diseases for South Lancashire Health Authority, said: "I have written to GPs in South Ribble advising them there may be people with cryptosporidium coming to see them."
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