H1N1 cases are rising in several parts of India, with doctors warning people to pay attention to persistent or worsening flu-like symptoms as the country enters its monsoon influenza season.

Delhi has recorded 1,449 H1N1 cases as of 12 August, according to data from the National Centre for Disease Control, more than six times the 229 cases recorded during the corresponding period last year.

The increase has also been reported in Mumbai in western India and Bengaluru and Chennai in the south, although the pattern is not uniform across the country. In Bengaluru, paediatricians have reported a rise in influenza A infections among children, with some cases progressing to pneumonia and respiratory distress.

H1N1, commonly known as swine flu, is a subtype of influenza A. It is a contagious respiratory infection that can cause symptoms ranging from fever and cough to more serious complications such as pneumonia and respiratory failure.

So what are the symptoms to look out for, and when should an apparently ordinary bout of flu become a reason to seek medical help?

Sudden fever and chills

A sudden fever is one of the symptoms commonly associated with H1N1, particularly when it appears alongside other flu symptoms. Patients in the capital have recently been presenting with high fever, cough, headache, sore throat, and other respiratory symptoms.

“Many of them have high-grade fever, which has been running for a few days,” Dr Atul Kakar, chairperson of internal medicine at Sir Ganga Ram Hospital, told ANI.

Cough, sore throat, and a blocked or runny nose

A persistent cough, sore throat, and runny or blocked nose are also common symptoms. Some people may also experience gastrointestinal symptoms, including vomiting and diarrhoea.

Body aches and extreme weakness

Body aches, headache, and pronounced tiredness can accompany H1N1. Dr Sandeep S Reddy, assistant professor and consultant in infectious diseases at Ramaiah Memorial Hospital in Bengaluru, told NDTV that H1N1 can cause severe body aches, headache, and “extreme fatigue or malaise”.

Difficulty breathing or chest pain

Most H1N1 infections are mild, but influenza can sometimes progress to pneumonia or respiratory failure. Difficulty breathing, shortness of breath, and persistent chest pain are among the warning signs that require prompt medical attention. Doctors in Delhi-NCR have also reported more serious respiratory complications, including falling oxygen levels, among some patients.

Dr S Chatterjee, an internal medicine specialist at Apollo Hospital, told The Indian Express that while people across age groups had been affected this year, some patients had progressed to respiratory failure.

Other warning signs include severe dehydration, confusion, or symptoms that become worse after initially improving. In Bengaluru, doctors have also reported severe dehydration among some children, with some requiring hospital treatment, respiratory support, and intravenous fluids.

H1N1 can look like a common cold

H1N1 symptoms can overlap with those of a common cold, particularly at the beginning of an infection.

Dr Reddy told NDTV: “On day one, it’s often hard to say,” because both illnesses can begin with a blocked or runny nose, sore throat, sneezing, mild cough, tiredness, and headache. “The distinction tends to come from how quickly things change, not from the symptoms themselves,” he said.

Influenza is more likely to involve a sudden onset and more pronounced fever, body aches, and fatigue, while sneezing and a runny nose are more commonly associated with a cold.

However, symptoms alone cannot confirm H1N1, so doctors may decide whether testing is necessary based on the patient’s symptoms, clinical condition, and other risk factors.

Who is most at risk?

H1N1 can affect people of any age, but some groups are more vulnerable to severe illness. Dr Chatterjee told The Indian Express that people with weakened immunity and conditions including diabetes, hypertension, lung disease, and heart disease, as well as pregnant women and older people, face a higher risk.

Young children are also considered vulnerable, and those with asthma, hyperreactive airway disease, cardiac conditions, allergies, and compromised immunity were at greater risk of complications.

In Bengaluru, Dr Rakshay Shetty, clinical director and head of paediatric services and paediatric intensive care services at Rainbow Children’s Hospital, told Times of India that his hospital had seen H1N1 particularly frequently among children in recent months. He said some children had developed pneumonia and, in rare cases, acute necrotising encephalopathy of childhood, a serious neurological complication.