With how stress-inducing, infuriating, exhausting, demoralizing, and much more negativity-inducing my depression is, it's a miracle I didn't snap at them. (Believe me, I wanted to, but while the venom within me might be tangible through nonverbal cues even in spite of me being autistic, I never lash out at work.) But here's what I wanted to say to them: depression is compared to sickness a lot, and not without justification.
Think about the absolute worst sickness you've ever had. Probably a flu of some kind. You were miserable. You could feel it throughout your whole body. Your life might have even been in danger! Depression of this magnitude is no different; it induces the exact same thing, except for two critical factors.
First, the risk of life-ending results is entirely self-inflicted, yet no less dangerous. If anything, it's actually a far greater risk! Statistically speaking, I'd have to check to confirm, but I'm pretty sure sufferers of this disease die more as a direct cause of it (be it suicide, alcohol poisoning, drug overdose, or similar) than any flu ever could.
Second, because it's not technically tangible to others (even though it can be felt--very much so, it can be physically affecting your health), because it's not contagious, rather than being legally forbidden from working (as with the case of a flu), I'm legally OBLIGATED to work. In a normal job, sure, I could call in sick, but for lifeguarding, the rules are different; I can't, especially on such short notice.
Even though I have one of the worst illnesses ever known, because it's not an accepted reason for not doing my job, I have to do my job. But I really, really don't want to. If I could skip work today, I would have. Trust me, with how miserable I'm feeling today, with how sick my non-physical sickness is making me, I want to not be here, yet here I am anyway.
Dead serious. This is something I can feel. In my chest, the raw power of it. I wish there were words in our language to explain this pain. But...it's not in my head. It's not purely my emotions. It's tangible. It exists in my body. It's that strong. It really is manifesting like an actual disease would. I try to explain this to people who don't know depression, or think they do but haven't experienced this. But the only way to truly know it is to have lived through this, and believe me...that's no easy task.