Two women of a family died of an unknown disease in the last three days in Baliadangi upazila, while three others of the family have been suffering from the same disease.
The deceased are Mina Begum, 35, wife of Hafizul Islam of Sangaon village, and Hafizul's elder brother's wife Pashina Begum, 40.
Three other victims -- Hafizul's mother Hazera Khatun, 54, younger sister Tanjina Akhter, 14, and younger brother's wife Aleya Akhter, 30, is now under close observation at Thakurgaon Sadar Hospital.
Thakurgaon Civil Surgeon Mohammad Mahfuzar Rahman said a medical team already visited the victims' house.
Meanwhile, a four-member team of experts led by Dr Omar Quaium of Institute of Epidemiology, Disease Control and Research (IEDCR) are on their way to Thakurgaon to assess the situation, the civil surgeon said, adding that, they were advising villagers to use mask until the disease is identified.
Hospital sources said Hafizul's wife Mina died on Friday night while Pashina passed away on Sunday.
Dr Abul Kasem, medical officer at Baliadangi Upazila Health Complex, said Mina was brought to the hospital with vomiting complaint and she was also behaving abnormally.
After taking treatment, family members took her home on Friday, but she died the same night.
In February last year, five people of a family died and three others fell sick at Bhandardaha Morichapara area in the same upazila.
Later, a team of IEDCR collected samples and found one of the five victims were infected by Nipah virus.