When you hear the phrase "chronic disease," it likely conjures up images of a sweet old granny baking cookies or knitting despite her arthritis. The thing is, chronic disease -- "a long-lasting condition that can be controlled but not cured" -- isn't limited to just the elderly.

Superstar Selena Gomez, who recently spoke up about her lupus diagnosis and undergoing chemotherapy treatment, is just one example of the millions of people who live with chronic conditions. These include things like arthritis, multiple sclerosis, diabetes, inflammatory bowel diseases and more.

Though chronic disease is more common in older adults, teens aren't invincible. Certain chronic conditions can develop at any age. Many of these illnesses are also invisible, meaning that you can't tell someone has one unless they explicitly tell you so. Here's what you need to know:

Their disease is not imagined...

If someone tells you they have a chronic disease, you might think something along the lines of, "Oh, but they don't look like they're sick." That's why chronic diseases are often referred to as invisible. Don't call your friend a hypochondriac or, worse, imply that they're imagining or making up their symptoms for attention. Their illness is a very real thing.

...and illness isn't always obvious.

Don't dismiss their symptoms or pain simply because you can't see any physical evidence -- a cast, a cane, you get the picture.

The Spoon Theory

No, not that kind of spoon. Christine Miserandino originally coined The Spoon Theory on her website But You Don't Look Sick. It's a simple analogy to explain what it's like to live with a chronic disease or disability. Miserandino first used it to talk about her experience with lupus.

The basic premise is that when you have a chronic illness, you wake up each day with a certain number of spoons. Every time you exert effort -- by getting out of bed, showering, eating, all those little things -- you lose a spoon. When you run out of spoons, that's it, day's done. It's an analogy that demonstrates the loss of control someone experiences when they're living with a chronic condition.

(I can't do justice to Miserandino's full explanation of The Spoon Theory -- seriously, read it here.)

They really, really can't make it out tonight.

Not enough spoons. Maybe bring the party to them instead? #FOMO

Having a chronic disease is not the same thing as having a cold.

You have good intentions when you say something like, "Oh, I've also been feeling kinda sick," but those words don't always convey the empathy you intend them to.

The thing is, colds -- or a fever or period cramps or just feeling less than spectacular for whatever reason -- are annoying for a few days, maybe a few weeks, but they're always temporary. Chronic disease most definitely isn't. Sometimes the symptoms worsen for a period of time -- these are generally called "flares" -- but the very definition of a chronic disease is that it's long-term. Many chronic illnesses also don't have a cure, so this could be something a person lives with forever.

Talking about the future can be scary.

People living with chronic disease have big dreams for the future just like you. The difference is they never know when a flare -- which could mean months of regular hospital visits, bedrest or even surgery depending on the severity -- could come along and mess their plans up. Learning to embrace uncertainty is a scary thing.

Chain-smoking cigarettes around them is not cool to the nth degree...

When a person's body already seemingly rebels against itself and it's something that's completely out of their control, watching other people willingly destroy their health feels extra annoying.

...but they're not necessarily a health freak.

People living with chronic conditions are usually hyper-aware about how they treat their body, yes, but this doesn't mean they will judge you for eating an entire chocolate bar in one go. They're simply conscious about how their body reacts to certain foods or activities, and that's a good thing. Oh, and they probs eat just as much, if not more, chocolate than you do.

Let them pick the restaurant.

Some chronic illnesses are more sensitive to diet than others. With inflammatory bowel diseases, for example, high-fiber foods like raw veggies can aggravate symptoms -- so a salad place might not be the best choice for lunch. But everyone is different, and people with chronic illness learn which foods to eat in moderation or to avoid altogether.

Don't always make alcohol part of the plan.

Just like certain foods can trigger symptoms, so can alcohol. Some people avoid booze altogether. Some can drink it, but only in small amounts. In the months after a flare, drinking alcohol may be completely out of the question. Certain medications might make it impossible to drink as well. Again, it's different for every person. When in doubt, ask if tonight's the time to rage or the time to chillax on the couch.

"I can't, I have a doctor's appointment" is never a lie.

Doctor's visits are part of everyday life for people living with chronic disease. Going in for a checkup often means much more than just a few deep breaths while the doctor checks your heartbeat. Blood tests, CT scans, colonoscopies, etc., are sometimes required to make sure everything is functioning the way it should be.

Bodies change. Deal with it.

During a flare or even on a daily basis, all those meds can come with not-so-glamorous side effects. This could mean sudden weight loss or gain or acne popping up everywhere. Don't point out the obvious, OK?

Everything is AWESOME (just like in "The Lego Movie" song)

Seriously! People with chronic illness aren't suffering from their disease -- they're living with it. Some days are worse than others, sure, but nobody is perfect. Everyone has their battles to fight. Chronic illness just happens to be one of those things for many people.