Families are being urged to follow coronavirus rules after a "significant" number of cases in the Neath Port Talbot area were linked to indoor gatherings.
According to Swansea Bay University Health Board, 23 confirmed Covid-19 cases in and around Briton Ferry have been traced back to birthday celebrations and extended family members visiting each other indoors.
Indoor visits are currently not permitted under coronavirus rules, unless you are part of a support bubble.
It is believed the true number of people infected could be higher, and people living in the Briton Ferry, Cimla and Resolven areas who have coronavirus symptoms are being advised to get tested straight away.
Dr Keith Reid, director of public health at Swansea Bay UHB, said: “We’re aware of extended family and friends who have apparently been going in and out of each other’s homes, which is not currently allowed.
“Being so familiar with each other, there has also been little or no compliance with social distancing, wearing facing coverings, etc, which has increased the likelihood of the virus being passed on.”
Dr Reid also said it was "disappointing" that contact tracers experienced a lack of cooperation from some of those who tested positive.
Not sharing details of their contacts would had slowed investigations and allowed the virus to carry on spreading, he said.
Dr Reid added: “If you test positive, please be open and honest with the tracing team. The details remain confidential and are used for contact tracing purposes to stop the virus in its tracks.
“We know that the majority of people do stick to the rules, and we thank them for their patience and cooperation.
"But we are not out of the woods yet. The numbers of positive cases in the Neath Port Talbot and Swansea areas have started to go in the wrong direction again, and it’s everyone’s responsibility to ensure they come back down.
“The rules are there for a reason – to safeguard people and slow the spread. A significant number of cases now are down to household gatherings, which should not be happening in the first place.
“You might think that because it’s your family member you’re meeting up with that it doesn’t matter, you couldn’t infect each other. But that’s wrong, and a totally false sense of security.
“The virus doesn’t care if you’re related. In fact, it thrives on close contact. Which is why we’re now dealing with this cluster in the Briton Ferry area.”
Last week, First Minister Mark Drakeford blamed ''household and social mixing'' for a rise in Covid cases in parts of Wales.