Prions are abnormally folded proteins that form toxic clumps in the brain. The illnesses they cause are rare and invariably fatal. (The most common prion disorder in humans, Creutzfeldt-Jakob disease, kills about 500 people per year in the United States.) Sometimes the disease is passed down from an unlucky parent; sometimes it develops spontaneously, a fluke mutation; sometimes it is the result of contagion, with the problem proteins making their way into the body from a tainted cornea transplant, or a skin graft, or beef infected with bovine spongiform encephalopathy, also known as mad cow disease.1 Whatever the cause, once symptoms start, the prions do their work quickly and irreversibly. They tear through the brain and kill healthy tissue, leaving empty holes behind.

With the diagnosis in hand, the Vallabhs made the decision to take Kamni off life support. The family gathered around her for a final goodbye. Sonia had braced herself for the moment of her mother’s death but found that, after months of uncertainty, it came as a relief. This was partly because, once Kamni was gone, long-absent support flooded in. Losing a loved one to dementia is mysterious, unsettling. Death, on the other hand, is binary. We all know the social conventions—cards and condolences, a shared mourning display. Several hundred people attended Kamni’s funeral. “It’s that kind of town,” Sonia says. “It’s also who my parents were in that town.”

Kamni’s diagnosis had come as such a shock that Sagar, hoping for final confirmation, had requested an autopsy. A tissue sample was sent for testing to the National Prion Disease Pathology Surveillance Center in Cleveland. Meanwhile, Sonia and her husband, Eric Minikel, returned to their lives in Boston. Between visits to Kamni in the hospital, Eric had managed to finish a master’s degree in urban planning at MIT and got a job as a transportation analyst. By the summer of 2011, Sonia had completed a law degree at Harvard and joined a small consulting firm. The nightmare of Kamni’s death began to recede.

That October, the couple went back to Hermitage for a friend’s engagement party. Just before they headed to the airport for their flight home, Sagar pulled his daughter aside. As a doctor, he was well trained in delivering bad news, but Sonia had never seen him struggle like this before. The results of Kamni’s autopsy had come in, he said. She had succumbed to a prion disease called fatal familial insomnia. There was a 50-50 chance that Sonia had inherited it.

Sonia broke the news to Eric on the plane, and he sobbed the whole way back to Boston, as concerned flight attendants helplessly offered their services. “It was exceptionally hard to watch my dad have to tell me, and then exceptionally hard to then have to tell Eric,” she recalls. “The person who had it worst that day was my dad. The second worst was Eric. The third worst was me.”

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Almost immediately, Sonia decided that she wanted to be tested for her mother’s mutation. Her doctors, genetic counselors, and even some of her family members recommended against it. If a disease has no cure, their reasoning went, what’s the point in knowing? Isn’t ignorance bliss? But Sonia was adamant. “You really want to hope that you’re negative, but the fear that you’re positive keeps interrupting, and it’s a constant psychological dialog,” she says. “Once you know, you start to adapt. What you can’t adapt to is something that keeps changing shape on you.”

It took weeks, but Sonia finally secured a test. The results wouldn’t come in for two months, so she and Eric went on their long-postponed honeymoon in Tokyo. They never got over the jet lag and spent their nights wandering down side streets. The trip became a physical instantiation of their mental state: alone in a strange place, speaking only to each other.