In season 1 of “The Last of Us,” Frank (Murray Bartlett) battled a degenerative disease that affected his mobility. But what exactly was his diagnosis?

Neither Frank nor his romantic partner/caretaker, Bill (Nick Offerman), mentioned his illness by name during season 1, and that was by design.

“We didn’t necessarily want to specify [the illness] for the audience. It was either MS or early ALS, but it was a degenerative neuromuscular disorder,” series cocreator Craig Mazin said during a 2023 episode of the official “The Last of Us” podcast.

MS stands for multiple sclerosis, an autoimmune disease that damages the insulating covers of nerve cells in the brain and spinal cord. Like Frank in “The Last of Us,” people with MS can become wheelchair-bound and experience muscle stiffness and tremors.

ALS, also known as Lou Gehrig’s Disease, stands for amyotrophic lateral sclerosis. Unlike MS, it is considered a terminal illness. The symptoms include trouble walking, hand weakness, and muscle cramps and twitching. ALS is a progressive disease, meaning that symptoms worsen over time.

As if living with a chronic illness weren’t challenging enough, Frank’s symptoms began after an outbreak of a fungal infection that turns humans into zombie-like creatures. His disease was unrelated to the pandemic.

In episode 3 of “The Last of Us” season 1, Frank, who lives with Bill in a fortified home cut off from the rest of the world, decides that he’s ready to die.

“What if somebody shows up who can help?” Bill asks.

“Who’s coming, Bill? The door to door MRI salesman?” Frank sarcastically replies. “There wasn’t anything to cure this before the world fell apart.”

Frank’s comment could point to either MS or ALS, as neither disease has a cure. Current MS treatments aim to reduce inflammation and resulting symptoms, while the goal of ALS treatment is to slow the progression of the disease and improve symptoms. Frank likely had no access to treatments in his post-apocalyptic world.

While it’s unclear whether Frank had MS or ALS, ALS seems like the slightly more likely diagnosis. MS is twice as common in women as in men, whereas ALS is 20 percent more common in men than women. Frank’s sense of hopelessness also points to a belief that he had a terminal illness, and while MS has no cure, it’s not fatal.

“My wife and I (both in healthcare) felt [Frank’s illness] resembled ALS more than MS,” one viewer wrote via Reddit.

Frank chooses to go out on his own terms in episode 3. After marrying Bill in their living room, Frank drinks a lethal dose of sleeping pills in a bottle of wine. Bill then reveals that he also put pills in his own drink, not wishing to live without Frank.

Bill and Frank’s ending in the TV series is different from their story in the “Last of Us” video game. In the game, Frank gets bitten by an infected person and decides to take his own life before succumbing to the disease. He and Bill don’t end on good terms, either; Frank leaves Bill a note expressing his unhappiness with him and his desire to leave their town.