A nip in the air at dawn but sweltering heat at dusk. The change-of-season weather is welcoming all viruses to multiply — and mutate.

The March malady list is growing alarmingly with hospitals and clinics across the city attending to patients suffering from severe cold, fever, chicken pox to diarrhoea.

And thanks to the mutating viruses, an increasing number of patients are arriving with symptoms that are at times misleading even doctors.

“For instance, what seems to be measles is turning out to be Rosealar infection on closer examination,” explains Jayati Sengupta, a city-based paediatrician. “The symptoms of both diseases are largely the same, leading to wrong diagnosis in a few cases.”

Mumps, too, has a close variant. There have been instances of physicians mistakenly diagnosing inflammation of the parotid gland, the largest of the three major salivary glands, as mumps.

“The pattern and prevalence of the diseases are changing because of the shift in seasonal cycles,” pointed out Sudarshan Chakrabarty, a senior physician at Sambhu Nath Pandit Hospital.

“Winter is now dragging into spring and monsoon continuing well into autumn. The viral strains are bound to undergo changes in such conditions... That is why we are witnessing malaria outbreak even in late November,” he explained.

Doctors are also noticing how the virus that used to affect only the upper respiratory tract is now hitting the lower tract, too, resulting in complications, particularly among smokers and those with a history of chronic obstructive pulmonary disease.

Another ailment of the season is viral diarrhoea, causing major fluid loss. The infection is on the rise among children, say doctors.

“A child suffering from viral diarrhoea has to be given adequate fluid support to prevent various complications,” said Mrinal Kanti Chatterjee, superintendent of BC Memorial Hospital for Children.

“The immunity against certain strains is on the wane. The strains that would earlier affect minors are now not sparing adults either,” observed Apurba Ghosh of Institute of Child Health.

Doctors say the best way to ward off viral attacks during this critical period of temperature swings is to ensure the immunity level does not go down. “Care has to be taken for right intake of nutrition and maintaining proper hygiene,” said a general physician.