The youngest member of the Oregon Zoo's elephant herd died suddenly last night.

Lily, a six-year-old calf, contracted a deadly virus earlier this week. The zoo announced her death shortly before 10 am today.

Lily suffered complications from endotheliotropic herpesvirus, a virus common to elephants that usually does not cause symptoms but can sometimes be deadly in very young animals, the zoo says. The virus is the leading cause of death in North America for Asian elephants younger than eight years.

"I can't imagine a more devastating loss for this zoo family and our community," Dr. Don Moore, zoo director, said in a statement. "Lily was the darling of the zoo. She was loved by everyone from her elephant family to the people who cared for her every day to her thousands of fans."

Lily's birth to zoo elephant Rose-Tu in 2012 was a tremendous PR coup for the zoo. But the happy news soured when The Seattle Times first reported that the zoo had signed away ownership of Lily to California-based traveling elephant show, Have Trunk Will Travel.

The zoo eventually paid $400,000 to Have Trunk Will Travel to purchase Lily and Rose-Tu, pledging they would live out their days at the zoo and its expanded elephant enclosure, Elephant Lands.

Lily's death could reignite controversy over keeping elephants in captivity—long a bitter debate among Portlanders. Animal rights activists were already planning protests at the zoo entrance for this weekend. This morning, they announced they'll hold a candlelight vigil for Lily on Saturday night.

"We are heartbroken to hear of Lily the baby elephant's passing at the Oregon Zoo today," said Lacey Kohlmoos, who organizes the animal rights group Care2. "No animal deserves to die in captivity."

Zoo officials say Lily's illness advanced rapidly.

Test samples sent to the Smithsonian's National Elephant Herpesvirus Laboratory showed that the virus had become active on Wednesday, Nov. 28. The next morning, Lily began acting lethargic and refused to eat, the zoo says. Zoo veterinarians gave the young elephant antiviral medication, fluids and a blood transfusion in an attempt to save her.

The zoo will be closed all day Nov. 30 and will re-open Saturday morning.

"Our staff did everything they could and fought to save her until the very end," Moore said. "Everyone is in mourning here. It is just heartbreaking."