The strain of hepatitis A that has infected 74 people on Oahu appears to be unique to Hawaii and has sent a third of its victims to the hospital in the biggest outbreak the state has seen in well over a decade.

Dr. Sarah Park, state epidemiologist, said the state sent specimens to the U.S. Centers for Disease Control and Prevention, which confirmed that the infectious liver virus that is felling so many people here is all one strain, as her team suspected. But it is a mysterious one.

“The bad news is that they have not been able to match it to any other strain in their bank,” Park said in an interview Thursday. “The subtype is commonly seen in other hepatitis A outbreaks, but the actual strain that they have sequenced is unique to Hawaii.”

In trying to figure out the source of the disease, sometimes knowing the viral strain can help. For example, the frozen berry mix that in 2013 sickened 165 people nationwide, including eight in Hawaii, was traced to contaminated pomegranate seeds from Turkey, partly because the main viral strain in that outbreak was common in the Middle East but not in the United States.

Over the past week, 22 new patients were identified with hepatitis A in Hawaii, lifting the total to 74 for this cluster of cases. The last time so many people had hepatitis A in the state was in 1998, when 148 people came down with it in the course of a year, Department of Health records show.

Outbreaks have dropped dramatically across the country with the advent of vaccines, now routinely given to children. Over the past decade hepatitis A cases have averaged 10 a year in Hawaii.

The contagious liver virus is shed in the feces of infected people and can spread through contaminated food or water, or close personal contact. Even tiny traces — just 10 viral particles — are sufficient for it to spread, Park said. Thorough hand-washing is crucial after using the toilet and before handling food.

Symptoms include fever, nausea, vomiting, diarrhea, fatigue, dark urine, jaundice and pain. Most people recover from the disease, but in rare cases it can cause liver failure.

Victims of the current outbreak range from 18 to 70 years old and are scattered across the island, from all walks of life.

The disease has a long incubation period, taking 15 to 50 days to produce symptoms. That makes it hard to trace the source of infection, because people have trouble recalling what they ate and drank over such a long period. The onset date for symptoms now stretches from June 12 to July 14.

“The longer this goes on, the more likely the contaminated item could be a frozen or a dry product that is something that has a long shelf life,” Park said. “It’s a commonly consumed, widely distributed item. Unfortunately, we don’t know what it is.”

Two of the victims are Maui and Hawaii island residents, but they were living on Oahu for a couple of months when they fell ill, Park said. They were no longer infectious when they returned to the neighbor islands.

Had their visit to Oahu been shorter — say, a week — it could have helped investigators narrow down possible food or drinks they might have consumed.

Along with proper hand-washing, Oahu residents are advised to talk to their health care provider about getting vaccinated against the disease. The vaccine, given in two doses six months apart, is good for life. It is usually covered by insurance.

MinuteClinics, which are walk-in clinics open daily in nine Longs Drugs stores across the island, are reporting brisk business.

“It’s very busy, very busy,” said Rachel Lee, senior practice manager for the MinuteClinics. “There is high demand. We run out occasionally, but by the next day we have it back in stock. We’ve ordered up at all the clinics so that we have enough vaccine on hand.

“I think because we still haven’t identified the source, it’s concerning to people,” she said.

Health Department investigators are pursuing all leads and interviewing patients in some cases multiple times as they uncover new information and try to trace the source of the outbreak. Park expressed gratitude for their cooperation.

“We keep hoping that at some point we’ll see something,” Park said. “We are invested in trying to resolve this. This is our home. We want this to stop.”