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.
Of these, 235 cases were in babies under three months and there have been six related deaths in infants up to the end of July, compared with five in the whole of 2008.
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."
The Department of Health's Joint Committee on Vaccination and Immunisation (JCVI) is considering the most effective ways to tackle the ongoing outbreak and a number of options are under review.
These include the introduction of a booster dose in teenagers and protecting newborn babies by either vaccinating them and their families and/or women during pregnancy.
Dr Amirthalingam added: "GPs have also been reminded to report cases quickly and been made aware of the HPA's guidance to help reduce the spread of infection - this improved awareness may be contributing to the increase in numbers of laboratory confirmed cases."