For more than a week now, children in some districts in the northern Indian state of Uttar Pradesh have been waking up with a high fever and drenched in sweat.
Many of them complained of joint pains, headaches, dehydration and nausea. In some cases, they reported rashes spreading across legs and arms.
At least 50 people, mostly children, have died of the fever, and several hundred have been admitted to hospital in six districts in the eastern part of the state. None of the dead tested positive for Covid-19.
At a time when India appears to be slowly recovering from a deadly second wave of coronavirus, the deaths in Uttar Pradesh have provoked a rash of panicky headlines about a "mystery fever" sweeping through the countryside of India's most populous state.
Physicians in a few of the affected districts - Agra, Mathura, Mainpuri, Etah, Kasganj and Firozabad - believe dengue, a mosquito-borne viral infection, could be the main cause of deaths.
On the frontline of India's encephalitis war
They say many of the patients were taken to hospital with a declining platelet - a blood component which helps form clots - count, which characterises a severe form of dengue.
"The patients, especially children, in hospitals are dying very quickly," says Dr Neeta Kulshrestha, the most senior health official of Firozabad district, where 40 people, including 32 children, have died in the past week.
Transmitted by female mosquitoes, dengue is mainly a tropical disease and has been circulating in India for hundreds of years.
It is endemic in more than 100 countries, but some 70 percent of the cases are reported from Asia.
There are four dengue viruses, and children are up to five times more likely to die during a second dengue infection than adults.
The mosquito - Aedes aegypti - breeds in and around houses in containers holding fresh water. "Humans provide breeding places and only humans can take them away," says Dr Scott Halstead, one of the world's leading experts on viruses spread by mosquitoes.
Nearly 100 million severe cases of dengue - severe bleeding, impaired organs - are reported worldwide every year.
"The combined impact of Covid-19 and dengue epidemics can potentially result in devastating consequences for the populations at risk," according to the World Health Organization (WHO).
Yet it is still not clear whether an epidemic of dengue is alone responsible for the fever-related deaths in Uttar Pradesh.
A state with more than 200 million people and traditionally poor standards of sanitation, high levels of-malnutrition in children and patchy health care routinely reports such "mystery fever" cases after monsoon rains every other year.
Outbreaks of the mosquito-borne Japanese encephalitis - first identified in Uttar Pradesh in 1978 - have claimed more than 6,500 lives since.
The disease mainly spread through Gorakhpur and adjoining districts which border Nepal in the foothills of the Himalayas, all low lying and prone to floods and providing a breeding ground for mosquitoes which transmit the virus.
A vaccination campaign, which began in 2013, has led to a drop in cases, but children continue to die.
Seventeen children have died of Japanese encephalitis in Gorakhpur so far this year, and 428 cases have been recorded.
In 2014, reacting to increasing cases of children dying of encephalitis and myocarditis - inflammation of the heart muscle - scientists examined 250 afflicted children in Gorakhpur. They found that 160 of them had antibodies to the bacteria that caused scrub typhus.
Scrub typhus, also known as bush typhus, is a bacterial infection spread through bites of infected viral mites.
The mites settle on the thriving vegetation in the villages after monsoon rains. Scientists found the mites on firewood which villagers store inside their homes.
A lot of the time, scrub typhus spreads when children handle firewood at home or defecate in the open in the bushes infested with the mites.
In a separate study, scientists also found scrub typhus and dengue were mainly responsible for post-monsoon fever cases in six districts in eastern Uttar Pradesh between 2015 and 2019.
Another potentially fatal bacterial infection called leptospirosis transmitted from animals to humans, and chikungunya, a mosquito-borne disease, accounted for the other fever causing pathogens.
"So there was a bunch of fever-related diseases happening in the region after the monsoons ended. You need systemic surveillance to keep a track of these diseases and treat them," says V Ravi, a professor of virology at the National Institute of Mental Health and Neuroscience (Nimhans), who led the second study.
Is India in the throes of a fever outbreak?
Earlier, in 2006, scientists investigated another "mystery" outbreak of fever-related deaths among children in Uttar Pradesh.
This time they found that the children had died after consuming cassia beans, which grew abundantly in the western part of the state.
This food poisoning was the result of "poverty, hunger, lack of parental supervision, ignorance, children playing by themselves, non-availability of toys and easy access to the plant", the scientists concluded.
Clearly, only more investigations and genome analysis will reveal whether the latest spate of "mystery fevers" in India are triggered by dengue alone, or a host of other diseases.
This would mean training local clinics and hospitals in collecting samples of people suffering from fever and send them for genome testing to labs.
Also, there is no clear record of how these fevers began and progressed; and whether the severity of the condition was determined by the long, arduous journeys that people have to make to government hospitals for treatment.
Or whether the afflicted children were suffering from other conditions such as tuberculosis.
If the cause of the mystery deaths is dengue alone, it points to the government's largely ineffective anti-mosquito control programmes.
The intensity of transmission, according to Dr Halstead, can be only determined by antibody tests - called sero surveys - by age groups.
"If we don't investigate properly and regularly, a lot of things will continue to remain a mystery," says an Indian virologist, preferring to remain unnamed.