There are just days to "get on top" of rising infections in Rhondda Cynon Taff (RCT) before a lockdown is considered, council leaders have warned.
Health officials are already looking carefully at other parts of Wales, where the case rate is rising too fast.
It follows the local lockdown imposed in Caerphilly.
Visits to RCT care homes have already been stopped, while six supermarkets have been given improvement orders after check-up visits by officials.
There have been warnings over social distancing due to small clusters around Porth, Penygraig and Tonypandy.
It has so far been a different nature to the infections in Caerphilly, where there has been community transmission.
The case rate looks at how many positive tests are recorded over the previous seven days.
For RCT, that rate has seen 32.7 cases per 100,000 over the last week.
Caerphilly was put into lockdown measures when it passed 54 cases per 100,000.
Updated figures of 72.9 cases per 100,000 now make it one of the hardest hit areas in the UK over the last seven days.
RCT is now seeing numbers of cases not seen since May.
Council leader Andrew Morgan said: "We don't need to panic just yet... but if the numbers do continue to grow as we've seen in Caerphilly, we do run the risk of a local lockdown."
He said the public had an important role to play in following advice like hand washing and social distancing.
"The only way this virus is spreading is because people out there are spreading it amongst each other, we have to take responsibility and try and control this," he added.
Mr Morgan said the council had had hundreds of complaints about supermarkets and on visits to 45 stores, officers had found some were "lacking and potentially adding to the risk".
The authority has given six supermarkets 48 hours to make improvements or face enforcement action, including the possibility of receiving closure notices.
A further eight supermarkets will receive follow up visits, said Mr Morgan.
Health officials have found the age category of 20 to 39 is where the overwhelming number of cases are.
RCT will be particularly worried if the transmission of cases passes to older and more vulnerable people in the area.
It has the highest death rate for Covid-19 in Wales in the pandemic so far, although numbers of deaths have dramatically slowed down.
There have been two deaths in care homes in the last six weeks and the last registered deaths in hospitals involving coronavirus were seven weeks ago.
There have also been very few, if any, patients on ventilation or in intensive care across the local health board in recent weeks.
Cwm Taf Morgannwg health board also had its lowest numbers of patients with Covid-19 in local hospitals in the most recent figures - with only one confirmed case, 14 suspected cases and 19 patients in recovery.
But it is not just RCT being monitored closely.
Infections have also risen to nearly 35 cases per 100,000 in neighbouring Merthyr Tydfil in the past week while Powys's case rate is also now well into double figures.
The case rate in Cardiff is still in double figures but has not taken off despite worries over the last couple of weeks.
Health experts were expecting clusters of infections as lockdown measures eased but it is the speed of the rise in positive cases which has been most worrying.
Public Health Wales (PHW) said of the RCT cluster that it was working with the council and Cwm Taf Morgannwg health board on a planned response.
"Test, trace, protect teams are working with those affected and contacting those people who have been in contact with a person who has tested positive," said Giri Shankar, PHW incident director.
"A proportion of the positive cases in this incident are from the young adult age group and, disappointingly, some have been reluctant to share details of where they have been and who they have been in contact with."
Officials are able to map the infections at a very local level to identify problems.
Mr Morgan said there was a "narrow window" of a few days, perhaps a week, to bring the virus under control before the area heads for a lockdown.
He confirmed a temporary testing centre was due to open in Porth from Thursday due to the rise in cases, particularly in the lower Rhondda area.