Update (September 7): CBS News reports that Indian health officials have identified both scrub typhus and dengue as the cause of these illnesses. More than 100 people in the Indian state of Uttar Pradesh have died in the last two weeks, and thousands have fallen ill.

In addition to the ongoing COVID-19 pandemic and an uptick of malaria cases across the country, an unknown disease has killed more than 50 people in the state of Uttar Pradesh, India in the last week; most were children. All of the patients tested negative for COVID-19, and India Today reports it is likely that scrub typhus is to blame, though other possibilities, such as dengue, have not been ruled out.

Uttar Pradesh (UP) is one of the most densely-populated areas of India, with more than 200 million residents inside an area roughly the size of Michigan. The area has recently been pummeled by another wave of COVID-19, The Hindustan Times reports, and monsoon season has brought a greater number of mosquito-borne illnesses. Now, many regions in the state are reporting illness and death due to a mystery fever.

The Independent reports Manish Asija, a lawmaker in the Firozabad region of UP, where a number of these cases have occurred, speculates that “water-logging, [and] lack of sanitation and hygiene are the reasons behind the disease spread.”

“The patients, especially children, in hospitals are dying very quickly,” Neeta Kulshrestha, a senior health official in Firozabad, tells the BBC.

According to multiple news outlets, the hundreds of people hospitalized by the illness have experienced fever, headaches, joint pain, nausea, rashes, and dehydration. A drop in platelet count has also been observed in many fatal cases. Though many of these symptoms can occur in severe dengue cases, The Hindustan Times and others report that the likely culprit is a bacterial disease called scrub typhus.

Scrub typhus can occur after a person is bitten by chiggers infected with Orientia tsutsugamushi bacteria. According to the BBC, these mites live on plants that flourish after the monsoon rains, and they can hitchhike into people’s homes on firewood. The symptoms of O. tsutsugamushi infection largely overlap with what has been seen in the patients.

According to the US Center for Disease Control and Prevention (CDC), there are no vaccinations for scrub typhus and it should be treated with the antibiotic doxycycline. The agency recommends covering exposed skin to prevent chigger bites.