Ankylosing spondylitis (AS) is a type of arthritis. Because it’s much less common than osteoarthritis (OA) or rheumatoid arthritis (RA), people who have it end up answering a lot of basic questions — like “You have what?” and, “What’s that?”

When you encounter a friend, co-worker, or family member with a condition you know little to nothing about, it’s hard to know what to say. Making an uninformed comment, no matter how well-intentioned, could wind up annoying or even hurting them.

Here’s a guide to help you make more informed choices when you talk to someone with AS.

Nothing can invalidate your friend’s pain more than acting like their disease is odd or made up. That may happen to people with AS more often than not, given that just .2 percent to .5 percent of people in the United States have the condition.

Take a few minutes to read about AS on your smartphone before you blurt out, “What the heck is that?” If you don’t have time to learn about the disease, at least ask about it in a more considerate way. “I’m not familiar with your condition. Can you tell me more about it, and how it makes you feel?”

Some diseases have apparent symptoms; AS isn’t one of them. Pain, stiffness, and fatigue are invisible to all but the sufferer.

Though you can’t see the pain, trust the person with AS — it’s there. And that pain can be debilitating enough to prevent them from hanging out with you or doing a lot of other things they enjoy. So when they tell you they’re sore, or stiff, or tired, try to be sympathetic.

AS is a form of arthritis, but it’s not your grandmother’s arthritis. OA is the kind you get as you age and the cushioning shock absorbers between your joints gradually wear down. AS is an autoimmune disease, meaning your immune system attacks your body. It’s likely related to genes. It can start at any age — even in childhood.

And unlike OA, AS goes beyond the joints. It also can cause problems with other body systems — like eye inflammation and heart problems.

Nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen (Advil, Motrin) and naproxen (Naprosyn) are one treatment for AS. Yet they aren’t strong enough to relieve everyone’s pain. For AS, stronger, biologic disease-modifying drugs like tumor necrosis factor (TNF) blockers and interleukin 17 (IL-17) inhibitors are also part of the treatment plan.

Sharing the same condition name is likely the only thing two people with AS have in common. Everyone’s disease takes an entirely different course. For some, it’s extremely painful and debilitating. For others, it’s so mild that it barely affects their day-to-day life. Rather than compare your friend with someone else you know, ask how their AS uniquely affects them.

Unless you’re a rheumatologist, don’t offer medical advice to someone with AS. There are a number of established treatments that are proven to work. Getting someone to try a questionable herbal remedy or alternative treatment not only is unlikely to work, but also could be dangerous.

You can’t be sure of anything with an unpredictable disease like AS. Everyone with AS wants to be fine, and they’re probably doing everything they can to be fine. But there is no cure for the disease.

Rather than trying to predict a future you have no way of knowing, be supportive. For example, offer to help with the grocery shopping or cleaning on days when they don’t feel well.

It’s hard to make it to work when pain and stiffness are your daily norm. As a result, people with AS lose an average of 10 workdays each year. Staying home and watching TV might sound like fun, but it’s not so fun if you have to support yourself and you need health insurance. Studies find that about half of people living with AS who are of working age are at risk of losing their jobs. There’s no great comfort in job insecurity.

If it were possible to overcome chronic pain and fatigue by thinking about something else, AS would be cured.

Besides being a really unsettling question, this is unnecessary to ask; AS is not typically a fatal disease. It can be painful and sometimes disabling, but death isn’t heavy on the minds of people who have it. Rarely, complications affecting the heart or other organs can make the disease more serious, but there’s no reason why someone with the disease can’t live a long, happy life.