Christian author Rachel Held Evans in 2018. (Dan Evans)

The death of best-selling author Rachel Held Evans this past weekend came so quickly and mysteriously that it left many struggling to understand how such a young and vital person could come to her end after what seemed like such a routine diagnosis.

Evans was widely admired for her willingness to wade into theological battles over the role of women, science, LGBT issues and politics, and her death has led to an outpouring of tributes and support for her family.

The symptoms experienced by the 37-year-old writer do not fit into an easily explainable diagnosis, medical experts say. The first sign to the public that something was wrong came from her own April 14 tweet saying she was in the hospital for the flu, a urinary tract infection and an allergic reaction to antibiotics.

Five days later came this distressing public update to her followers from her husband, Dan: “During treatment for an infection, Rachel began exhibiting unexpected symptoms. Doctors found that her brain was experiencing constant seizures.”

Her husband said doctors had put her into a medically induced coma to stop the seizures while they tried to figure out what was causing them.

“Something enormously dramatic happened between April 14 and 19,” said William Schaffner, an infectious-disease professor at Vanderbilt University who stressed that he did not treat her or have firsthand information.

Schaffner and other medical experts stressed that with the limited information known about Evans’s case, it is hard to tell exactly what happened. But the fact that her illness began with flulike symptoms and then progressed to constant seizures strongly suggests she may have developed encephalitis, an inflammation of the brain.

With encephalitis, as tissue becomes inflamed, the brain can begin to swell and bulge against the skull bone, building up internal pressure, causing damage to the tissues and triggering abnormal electrical signals that cause seizures, Schaffner said.

In subsequent updates to Evans’s followers days later, her husband said that doctors were struggling to explain exactly what was causing the seizures, calling them “compounding factors.” When the doctors tried to wean her off the medicine keeping her in the coma, the seizures returned.

“When you hear about brain swelling and seizures, it does point to central nervous system infection and [the] possibility of encephalitis,” said Amesh Adalja, an infectious-disease and critical-care doctor at Johns Hopkins Center for Health Security.

The possible flu, urinary tract infection and antibiotic allergic reaction that Evans first tweeted about are not likely to have caused the encephalitis, Adalja said. Some antibiotic reactions can cause seizures, but those seizures would not have continued over time as Evans’s did.

Encephalitis is an especially perplexing disease. It can be triggered by a host of things, including bacterial infections and rabies. But the most common cause is viral infection, including viruses borne by mosquitoes, such as West Nile. Often there is no medicine to give a patient with encephalitis, Adalja said. Doctors simply try to manage the swelling in the brain.

The cause in many cases remains an enigma, Schaffner said. Many of those who recover are left with intellectual or motor-function disabilities because of damage and scarring to the brain that occurred during the inflammation.

“It’s difficult to believe that in the 21st century this is the case, but with encephalitis, it’s an illness that can sometimes affect young people with dramatic, unfortunate results,” Schaffner said.

In Evans’s case, according to her husband’s online updates, her doctors tried over several days to juggle the benefit of keeping her in a medically induced coma with the risks it posed.

On Thursday, her brain began to swell so much it caused severe damage, her husband said, resulting in Evans’s death Saturday morning. A family friend, Sarah Bessey, said there were no further updates from Evans’s family about her death.

The testimonials of hundreds of fans and followers on Twitter trended over the weekend with the hashtag #BecauseOfRHE.

Many women wrote that because of Evans’s writing about the place of women in Christianity and her encouragement, they became pastors or Christian authors. Or they held onto their faith despite doubts because she wrote so eloquently of her own.

Sarah Pulliam Bailey and Julie Zauzmer contributed to this report.