A common bacterial infection known as Strep A has likely killed over a dozen children in the UK in recent months. On Friday, health officials reported yet another fatality linked to the bacteria, which can, in rare cases, cause a more invasive and life-threatening infection. There are likely several reasons why the outbreak appears to be more severe than usual, though government experts have ruled out the possibility of a more virulent strain behind it.

At least 13 children in England have died within a week of developing a severe Strep A infection since September, according to an update released Thursday from the UK Health Security Agency (UKHSA), while two suspected deaths have been reported in Ireland and Wales. Officials with the UKHSA and the Brighton & Hove City Council jointly announced the latest suspected death Friday, though it’s not yet clear whether this death is in addition to the official tally, according to the BBC. At least 60 deaths overall have been reported in England.

Strep A, also known as group A streptococcus, are bacteria that can cause a variety of infectious diseases along different parts of the body, including strep throat, scarlet fever, and impetigo. Typically, these infections only cause mild illness or can be managed effectively with antibiotics. But every once in a while, they can cause much more serious trouble. The particular form of Strep A tied to these deaths is called invasive group A streptococcus (iGAS) disease, which happens when the bacteria (or their toxins) reach more vulnerable parts of the body like the bloodstream. Once there, the infection can rapidly trigger life-threatening symptoms like septic shock, organ failure, or necrotizing fasciitis (the “flesh-eating” disease)

Strep A outbreaks do occur regularly, but the typical season for it in the UK starts in the beginning of the year. This season is on track to be the most widespread and deadliest in recent years. The last severe Strep A season occurred between 2017 to 2018, which ultimately caused 355 deaths in total, including 27 deaths in children.

A 2019 study found evidence that the recent emergence of new Strep A strains contributed to large spikes of scarlet fever reported in the UK over the latter half of the 2010s. But according to the UKHSA, there’s no data suggesting that a new variant is responsible for the intensity or greater rate of cases seen this year.

Instead, they argue the rise is likely the result of “increased susceptibility to these infections in children due to low numbers of cases during the pandemic, along with current circulation of respiratory viruses.” This year has seen the return of respiratory infections like the flu and RSV in the UK and elsewhere, often in waves outside of their usual season. And it’s known that people can be more vulnerable to developing iGAS if they catch Strep A while sick with another infection like the flu.

Like the UK, the US has been seeing high levels of flu and RSV activity this fall and winter, while cases of covid-19 are currently lower than they have been during the past two winters but are increasing lately. And the Centers for Disease Control and Prevention is looking into whether there’s been an increase in iGAS cases among children in the U.S. this year as well.