A large number of people across the city are suffering from fever, cough and runny nose, doctors said.
Many people fell ill during or before the Puja. The large assembly of people in and around pandals could have helped spread the viral infections, a doctor said.
The symptoms of the infections are self-limiting and not leading to any severe outcome in most people. It is better to keep the elderly and the children protected from those with such symptoms, said doctors.
Paediatrician Apurba Ghosh said nearly 40 per cent of patients in his clinic were coming with complaints of fever and cough.
"I think the congregation of people in Puja pandals and other places during the Puja might have led to a spread," he said. "This is an ongoing viral infection. We are not suggesting a viral panel test in most cases because the test is very costly."
Chandramouli Bhattacharya, an infectious disease specialist at Peerless Hospital, said he had received calls from patients who reported symptoms like fever, runny nose and cough.
"Since the OPD was closed during the Puja, I received calls from some patients. They were worried about what to do, but in most cases, the symptoms were self-limiting," he said.
An official at Peerless Hospital said a few elderly patients were admitted during the Puja with chest infection following a viral infection.
A Bhowanipore resident fell ill just before the Puja. The woman, a resident of Rakhal Mukherjee Road, said she had been running a temperature from October 18. "I had fever till October 20. There was headache and weakness along with fever. A fatigued feeling is still persisting," she said.
Four people in a family of five in a south Kolkata neighbourhood fell ill in the last 10 days. "My parents, both in their 70s, fell ill first. Then my wife. Finally, I had fever on Dashami," one of them said.
Doctors said the most likely cause of the infections was respiratory viruses that become more active with fluctuations in temperature.
While the afternoons were warm over the past fortnight, those staying outside till late in the evening said they felt a nip in the air, especially in neighbourhoods where the population density is not so high.
A police officer who was on duty on all days of the Puja said he wore a full-sleeved uniform from Saptami. "One of the officers under my supervision started wearing a wind-cheater while driving his two-wheeler at night," he said.
On Navami and Dashami (Monday and Tuesday), the city received rains.
There was also a drop in Kolkata's air quality in the week leading to the Puja. The website of the state pollution control board shows that the air quality was "moderate" between October 14 and October 20. It improved in the next few days.
According to the Central Pollution Control Board, moderate air quality can cause "breathing discomfort to the people with lungs, asthma and heart diseases".
Bhattacharya said a drop in the air quality weakens the defense mechanism of the respiratory tract.
A doctor at a health clinic run by the Kolkata Municipal Corporation said they have been receiving a number of patients with such symptoms for the last few weeks. "Some of these patients are also suffering from symptoms triggered by seasonal allergies," the doctor said.