It started happening just before the bottom of the sixth inning began.

I caught the pitcher’s final warm-up as I stepped out of my crouch to throw down to second. I cocked my hips, transitioned the ball to my bare hand, and felt my insubordinate fingers lock onto the baseball, refusing to release it at the top of my throwing motion. The ball darted into the all-sand infield just left of the pitcher’s mound, skipping off toward where the shortstop would have been if he weren’t covering second, and rolling to a stop in short left field.

“My bad,” I yelled.

No one ever gets caught stealing at this level; it has happened maybe twice in three years of weekly play. Pitchers aren’t good enough at holding runners on, catchers aren’t good enough at blocking balls in the dirt or throwing to bases, infielders aren’t good enough at receiving throws and tagging runners. There are just way too many variables that could go wrong on the defensive side, and all the baserunner has to do is haul his ass 90 feet.

But a catcher with a strong or accurate arm can at least dissuade the casual basestealers — the fat guys, the hungover crowd, the smokers, and the one fat, smoking, hungover dude.

Last week, I caught 10 innings and my throws were sharp. Not hard, but on target, and good enough to limit only the speedy runners to taking bags when the situation called for it, instead of beckoning every runner to steal every time he reached base.

This week, after the errant warmup throw, the latter happened. This week, they ran wild, taking advantage as, with increased concentration on controlling my hand, my throws grew worse: pop-ups 15 feet to the left of second base, bloopers over the third baseman’s head.

I knew I shouldn’t have caught before I even arrived at the ballfields in Red Hook. The pain in my back and shoulders nagged me for days before, knifing into my neck and radiating down my arms into my hands.

No one here would judge me if, while we divvied up positions before the game, I grumbled something about my back acting up and begged out of catching. But when no one else immediately volunteered, I stepped up, knowing what I do about how much more value a slap-hitting, poor-defending backstop offers to his team than a slap-hitting, poor-defending corner outfielder.

I started playing pickup baseball in Brooklyn three years ago this month, and, coincidentally, just a few weeks after I first felt the symptoms of M.S.

The game started because a guy named Grant heard about adult hardball leagues that played around the borough, then got drunk and put up a Craigslist ad inviting players to Prospect Park to come try out for his team. When a bunch of people showed up the next day, Grant copped to having no idea how to get involved in any organized league but the group decided to break into two teams for a pickup game anyway. They played again the next week.

I heard about it from a couple of friends a week later, and I’ve been playing pretty much every week since, work and weather permitting.

Grant followed a girl to South America that winter. New leaders emerged, and slowly, the game became better organized: equipment purchased, vague bylaws and codes of conduct established. Eventually enough guys started playing regularly that we had to cap the roster and stop welcoming passing hipsters in skinny jeans and hiking boots, even though we all agreed that was kind of awesome. Fewer guys smoke cigarettes during play now, and more wear real baseball pants.

We even legitimized and secured permits for fields, though our disagreement with the Parks Department over the actual length of baseball season — they say April-to-Labor Day, we say March-to-Thanksgiving — means we still wind up itinerant for a few months of each year, playing at whatever Brooklyn diamond seems least likely to be overrun with flag football or LARPers or leftover temporary fences from a concert.

During that time, what started as some pain in my upper back gave way to a variety of stranger problems: numbness in my hands, tingling in my foot when I worked out too long, difficulty grabbing certain chords on the guitar, a buzzing sensation in my neck when I tilted my head downwards, and a few terrifying episodes in which I entirely lost control of my left arm.

It took five doctors, countless tests and over a year to get a diagnosis, then a five-day hospital stint for steroid treatment (which did nothing for my power!) and now a bevy of pills and vitamins and a weekly injection to reach some semblance of stasis.

I still have the pain — some days and nights worse than others — plus an odd hypersensitivity to uncomfortable seating arrangements and a Zoolander-like inability to turn my head all the way to the left. Sometimes the drugs leave me feeling a bit sick, light-headed, or just dumb. Plus there are the times when, if my body gets too hot or too tired, certain parts don’t seem to comply with my brain’s instructions, ever a strange sensation. That’s what was happening that Saturday in Red Hook.

But my doctor says the lesions on my brain and spinal chord that cause all those issues have stopped growing, and claims that an M.S. diagnosis is not the damning sentence it was even a few years ago. He says, with treatment, I should expect to remain at least this healthy into old age.

In other words, I have no reason to believe I’ll have to stop playing baseball anytime soon.

That’s important.

I’m lucky enough to say that the worst effect M.S. ever had on me was the pervasive uncertainty it unleashed. The symptoms of the disease can be so vague and potentially so comprehensive that it’s easy to become concerned that every little thing represents a symptom, every twitch and pain and hiccup, every lost memory and unrecalled word emblematic of the onslaught of sickness. It’s frightening.

Playing baseball helps keep that paranoia at bay. Being able to compete, even at a casual level, with a group of men who presumably do not have M.S. reminds me that the disease cannot have made all that much headway before the doctors stopped the progression. It’s not like I was ever that good at baseball in the first place, and I’m still decent enough now to mostly avoid embarrassing myself among a bunch of guys who played high school and college ball.

I’m conscious of the disease while I play, of course. There are rare humiliating moments like that inning behind the plate, and slightly less epic ones like just dropping a flyball in the outfield and wondering if I would have made the play if my fingers weren’t numb under my glove. But my errors, I’ve realized, are no more costly or common than those of plenty of other guys on the field. Stranger, perhaps, but not necessarily more egregious.

Sometimes I fantasize about what might happen if I could be magically freed of the symptoms of the disease — the knots in my back loosened, all feeling in my extremities restored — while maintaining all the new skills I’ve certainly developed to compensate, some great Harrison Bergeron unveiling. But I know that’s not to be, that who I am now is who I am. And I know, rationally, that it doesn’t really matter if I dropped that flyball because I have M.S. or I dropped that flyball because I’ve been a crappy defender my whole life, because both M.S. and crappy defense are now invariable parts of my constitution.

Playing with mostly the same group of guys for several years, you develop pretty strong scouting reports. I assume the others see me as a good contact hitter without a lot of power, and a poor defender occasionally prone to the yips. Other than the two guys that know me personally, they have no idea I have a decent excuse for an awful throw here and there. That’s fine. The last thing I want is pity or mercy.

And though most of our bench conversations focus on baseball, through the years I pick up more about the guys around me on the field and learn which guy needs surgery but lacks insurance, which guy runs the bases with a helmet because he fears a seizure, which guy is suffering through a brutal divorce, which guy was uprooted by Katrina, and I realize how silly I am, how selfish, to assume that I’m the only one here playing to prove something to myself, or to escape some rough reality.

With enough experience in baseball or life, we are doomed to endure a great deal of misfortune. That’s universal. Frozen ropes sometimes fly right into fielder’s gloves and loved ones sometimes die young. And we can harp on the awful things that seem to happen for no good reason, let them weigh us down and ruin us, or we can accept that they are likely random, the pitfalls of existence, and shoulder them as best we can and focus on the dribblers that squeak through the infield.

Right around the time my back started hurting, some guy got drunk and put up a Craigslist post. I am still playing baseball three years later.

Cool.

I am riding 30 miles on a bike on Sunday to benefit the National MS Society. Please donate if you can.