A rise in whooping cough cases has prompted a warning to parents to make sure their children's vaccinations are up to date.
The Health Protection Agency (HPA) said there have been 665 confirmed cases already this year, compared with 1,040 in the whole of 2011.
Cases have been reported across all regions in England with some areas reporting clusters in schools, universities and healthcare settings.
Whooping cough, also known as pertussis, affects people of all ages but the recent increase has seen cases in very young children, who have the highest risk of severe complications and death.
The infection in older people can be unpleasant but does not usually lead to serious complications.
The main symptoms of whooping cough are severe coughing fits which, in babies and children, are accompanied by a characteristic "whoop" sound as the child gasps for breath.
The infection can be treated with a course of antibiotics but young infants may need hospital care due to the risk of severe complications.
Dr Mary Ramsay, the HPA's head of immunisation, said: "Whooping cough can be a very unpleasant infection. Anyone showing signs and symptoms - which include severe coughing fits accompanied by the characteristic 'whoop' sound in young children but as a prolonged cough in older children and adults - should visit their GP.
"Whooping cough can spread easily to close contacts such as household members. Vaccination is the most effective way to protect people from this infection and uptake of the vaccine is very good.
"Parents should ensure their children are up to date with their vaccinations so that they are protected at the earliest opportunity. The pre-school booster is also important, not only to boost protection in that child but also to reduce the risk of them passing the infection on to vulnerable babies, as those under four months cannot be fully protected by the vaccine.
"The HPA has written to GPs to remind them of the signs and symptoms of this infection and stress the importance of vaccination. The agency is also encouraging GPs to report cases quickly and to make them aware of the HPA's guidance to help reduce the spread of the infection."
The HPA is also reminding parents to ensure their children are protected against measles after a slight increase in cases since the beginning of the year.
So far, 251 measles cases have been reported in 2012 compared with 200 cases in the same period last year. The majority of cases have been in people who had not been vaccinated.
More than half (136) of those cases have been reported in the North West region of England.
Dr Ramsay said: "As we approach the time of year when many children are travelling on school trips and family holidays, we are again urging parents to protect their children against measles by ensuring they have been immunised with two doses of MMR.
"This is particularly important given the increase in cases in children and young adults over the last few months.
"The continued outbreak in the Merseyside area is a clear demonstration that measles can be a very serious illness.
"Measles is a highly infectious disease which spreads very easily, particularly in schools and universities. It's never too late to get your child immunised with two doses of the MMR vaccine. We cannot stress enough that measles is serious and in some cases it can be fatal. Delaying immunisation puts children at risk."
A Department of Health spokesman said: "We continue to see high uptake of the whooping cough vaccine, but the increase in cases - while not uncommon with such highly infectious diseases - highlights the importance of vaccination against this and other illnesses. Parents should make sure their children are up to date with all vaccinations, and should speak to their GP if they need advice.
"The Health Protection Agency will continue to monitor levels of this disease very closely and the Joint Committee on Vaccination and Immunisation keeps all its advice on immunisations under review."