Japan's National Institute of Infectious Diseases (NIID) said it has confirmed the first domestic case of human-to-human transmission of severe fever with thrombocytopenia syndrome (SFTS), a tick-borne viral disease.

The viral infection was passed from a patient to a doctor, but the doctor's symptoms have improved, the NIID said earlier this week.

The doctor, a man in his 20s, attended to a patient in his 90s who had been diagnosed with SFTS after visiting the emergency room last April due to deteriorating health, the institute said.

The doctor performed various procedures on the patient, who later died, including removing his catheter post-mortem, during which the doctor reportedly wore a face mask and gloves but no goggles.

Approximately nine days after the patient's death, the doctor developed symptoms of fever and headache and was diagnosed with SFTS upon medical examination, it added.

The genes of the viruses in the two men were found to be identical, confirming human-to-human transmission, marking the first such instance in Japan, according to the NIID.