Doctors are monitoring an upsurge in cases of the illness in the county, where there have been as many cases so far this year as in the whole of 2007.

The condition is much less common than it was 40 years ago but can still make people very ill and two youngsters have needed hospital treatment.

The majority of those hit by the disease have been aged under five and have been suffering from a relatively mild form of the condition which is caused by bacteria that affect the throat. These go on to cause a rash on the body.

Family doctors have been urged to be on the lookout for cases which have been confirmed in laboratory tests in 18 patients. Symptoms can be confused with other conditions including measles.

Martin Schweiger, consultant in communicable disease control with the West Yorkshire Health Protection Unit, said: "We would expect to see a few cases of scarlet fever, especially at this time of year, but there are an unusually high number being reported in West Yorkshire at the moment.

"However, most of the cases we have had reported to us seem to be sporadic and not linked and are widely distributed across the county. We are aware of two cases which have needed hospital admission to date, but both of these have recovered well.

"It usually takes one-to-four days for the symptoms to appear after coming into contact with the bacteria and scarlet fever usually clears up after a week.

"For the majority of cases a course of antibiotics will cure the illness and reduce the risk of complications.

"Anyone suffering from scarlet fever should stay at home and not go to school or work for at least 48 hours once antibiotics have been started, to avoid passing the infection on to anyone else."

Figures show there were 355 suspected cases of scarlet fever across the whole of Yorkshire in 2007, up from 287 the year before.

The illness has become quite rare because of the regular use of antibiotics for throat infections.

It is most likely to affect children between four and eight, although people of any age can catch it, usually through bacteria passed on by coughs and sneezes.

The first symptoms often include a sore throat, headache, fever, nausea and vomiting. Between 12 to 48 hours after this, the characteristic rash develops, usually in those aged under 18.

This feels like sandpaper to touch and first appears on the chest and stomach before spreading to other parts of the body.

Other symptoms include swollen neck glands, a white coating on the tongue and general feeling of being unwell.

Dr Schweiger said sufferers with high temperatures should drink plenty of fluids, keep cool and take paracetamol to relieve discomfort and bring down temperatures.