A wave of mild fever, nasal discharge, severe headache, and congestion is spreading throughout the city.

A critical care specialist said that nearly half of his patients had these complaints. Doctors said that diarrhoea, caused by rotavirus infection, has been detected in adults.

The viral outbreak is affecting all age groups, including children. A paediatrician said similar symptoms, along with conjunctivitis, are common among children.

The temperature change — a nip in the air during mornings, even a week ago, to warmer mornings and days — always helps viruses replicate faster, said doctors.

An official from the India Meteorological Department stated that the minimum temperature has been in the range of 20-21 degrees Celsius, while the maximum temperature has been in the range of 30-31 degrees for the last three or four days.

Doctors are not advising tests in most cases, as signs and symptoms suggest these are viral infections. The treatment is supportive, a doctor said.

A paediatrician stated that among the limited tests conducted by parents of children, adenovirus has been detected in the samples.

“Between 40 and 50% of patients in my clinic are those who are coming with complaints of low-grade fever, severe cough, throat pain and runny nose. About a fortnight ago, about 20% of patients in the clinic had such complaints,” said Amitabha Saha, director of critical care medicine at Desun Hospital.

“Elderly patients are also getting admitted to the hospital with complaints of respiratory failure,” he said.

A south Calcutta resident had to skip an important work-related meeting on Saturday after he fell ill. “I had a low-grade fever, but a splitting headache. There was a complete loss of appetite, and rashes also appeared on my skin,” said the man.

He consulted a friend, who is also a skin specialist. “He said it was a viral fever and asked me to take rest. I had a severe headache on Saturday. I am still very weak and finding it difficult to speak,” he said.

Chandramouli Bhattacharya, an infectious disease specialist at Peerless Hospital, said he has been treating patients with similar complaints for several months now. The number of infected patients rose during the peak of winter when air pollution was high, he said.

Bhattacharya, however, added that a few patients were admitted to the hospital with severe diarrhoea. Tests revealed that they were infected with rotavirus.

“I treated three patients recently, all of whom were admitted with severe diarrhoea. Two of them underwent tests that showed the presence of rotavirus in their samples. The third patient had recovered considerably and did not need a test,” Bhattacharya said.

According to doctors, rotavirus infections predominantly affect children. The rotavirus vaccine was incorporated into India’s universal immunisation program in 2016.

Paediatrician Apurba Ghosh stated that a significant number of children are becoming infected, with over 30% also suffering from conjunctivitis.