People in NSW are being warned to be alert to the symptoms of meningococcal disease after the death of a third person in the state this year.

NSW Health says there have been 29 cases reported this year, the majority due to meningococcal B strain of the infection.

A woman in her late teens died recently from the disease and NSW Health is warning it can be fatal within hours if left untreated.

Meningococcal disease is now uncommon due to vaccination but there has been a slight increase in cases in recent weeks, compared with the same period over the previous five years.

The disease can occur in people even if they have been vaccinated and children under five and 15 to 25-year-olds are at the greatest risk of contracting the infection.

NSW Health's Jeremy McAnulty says early intervention can be lifesaving.

"Meningococcal disease symptoms can appear suddenly and become very serious very quickly.

"I urge everyone not to discount symptoms when they appear or assume it may be just a mild infection. If you suspect meningococcal disease, don't wait for the rash - see a doctor immediately," Dr McAnulty said in a statement.

The symptoms include: severe, unexplained limb pain, difficulty waking up, high pitched crying in babies, severe headache, aversion to bright lights, stiff neck and a red-purple rash which doesn't disappear when pressed with a glass.

The rash does not always occur, or may present late in the illness.

"If symptoms rapidly worsen, or if your child is very unwell, call triple zero or go straight to your nearest emergency department," Dr McAnulty said.