Health experts have said they are concerned about a surge in the number of cases of the potentially fatal disease whooping cough, with more than 3,500 reported so far this year compared to 1,118 for the whole of the last.
The outbreak has mainly affected teenagers and young adults but high numbers of cases have also been seen in very young babies, who are at highest risk of severe complications.
The Health Protection Agency (HPA) said 1,047 cases of whooping cough in England and Wales were reported to it in July, bringing the total number of cases so far this year to 3,523.
Whooping cough, also known as pertussis, affects all ages but is especially dangerous for young babies as they do not get the benefits from vaccination until they are around four months old.
The highly infectious bacterial disease comes and goes in a cycle, with increased incidence occurring every three to four years. The last peak was in 2008, when 421 cases were reported to the HPA between January 1 and June 30. Today's figures show a much more worrying picture in 2012.
Dr Gayatri Amirthalingam, an expert in immunisation at the HPA, said: "We are very concerned about the continuing increase in cases.
"Parents should ensure their children are vaccinated on time so that they are protected at the earliest opportunity and be alert to the 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 or adults.
"We also advise parents to keep their babies away from older siblings or adults who have the infection."
A Department of Health spokesman said: "We continue to see high uptake of vaccination against whooping cough and are investigating the recent increase in cases. This highlights the importance of vaccination against this and other illnesses.
"The Joint Committee on Vaccination and Immunisation is looking at whether more people need to be vaccinated. Careful consideration is always needed around expanding any programme. Parents should make sure their children are up to date with all vaccinations, and should speak to their GP if they need advice."