Scarlet fever levels have begun to drop in England, but remain "substantially elevated" across the country.
Official figures show that the total number of cases so far were 8,322, the highest number since 1980, when there were 11,118 cases.
Public Health England (PHE) is working with laboratories to work out reasons for the current upsurge in cases.
Scarlet fever is a seasonal disease. This is the time of year when there is usually a decline in the number of cases. While data from recent weeks does suggest that the decline has started, the fluctuation indicates a continued need to remain vigilant, PHE warned.
Dr Theresa Lamagni, PHE’s head of streptococcal infection surveillance, said: “It is still too early to be confident that the drop in the number of cases will be maintained as we are seeing fluctuating numbers each week and incidence remains high compared to recent years. We will continue to monitor the situation closely to see if there is a sustained fall over the coming months."
While scarlet fever is usually a mild illness it should be treated with antibiotics to reduce the risk of further complications. It is mainly a childhood disease, most common between the ages of two and eight years, although adults can also develop scarlet fever.