A deadly strain of "flesh-eating bacteria" in Japan infected 525 people this year, according to the country's National Institute of Infectious Diseases (NIID). Scientists are reportedly unsure why there has been a sharp increase in the life-threatening disease in 2017 compared to the 203 infected people in 2013.

The latest tally, which was announced on Dec. 10, found an alarming number of patients were diagnosed with Streptococcal Toxic Shock Syndrome (STSS), including 66 in Tokyo, the highest of any prefecture. There have also been 40 infected patients in Kanagawa, 32 in Aichi, 31 in Fukuoka, and 28 in Hyogo, according to data compiled by NIID.

The disease is caused by a Streptococcus pyogenes, better known as group A streptococcus. Some of the illnesses caused by the bacteria are strep throat (a mild infection that occurs in children), scarlet fever (a contagious infection that is seen in children and adults) and Post-Streptococcal Glomerulonephritis (a kidney disease that occurs after throat and skin infection), according to Centers for Disease Control (CDC). 

Some of the initial symptoms caused by the bacteria are swelling and pain in the hands and feet and fever. However, the minor symptoms can grow deadly in a matter of hours, necrotizing muscle tissues at various parts of the body, leading to multiple organ failure and seizures. STSS, which primarily targets one’s blood and lungs, can cause death within a few days of contracting the bacteria.

Group A streptococcus is not the only strain of bacteria in Japan. Another form of the disease,  group G streptococcus, has also proven to be serious and dangerous.

“It is conceivable that a growing number of patients with STSS have been infected with bacteria other than group A streptococcus,” said Ken Kikuchi, a professor of infectious diseases at Tokyo Women’s Medical University, according to Japanese newspaper Asahi Shimbun.

Most patients affected by STSS are above 30 years of age. Symptoms of the disease can show up in patients with no record of any chronic illness. However, according to Kikuchi, elderly people who have been diagnosed with the disease needs to be treated with extra care.

“The signs of an STSS-infected area likely appear from the feet,” Kikuchi said. “The elderly should be careful about swelling of their feet and go see a doctor immediately when swelling appears.”

Although some medical experts suspect that the bacteria gain entry into the human body through infectious open wounds, the path of the bacteria mostly remains a mystery.

STSS has a fatality rate that may exceed 50 percent, according to the CDC in 2010. In early stages of the disease, before the bacteria have spread throughout the body, penicillin-based antibiotics can be given to the patient for a successful therapeutic treatment. In extreme cases, badly infected parts of the body can be amputated.

In November, U.K. health officials reported that nine people in the London area had died from a group A streptococcus outbreak, with 132 reported cases over the previous 18 months.