You are a worker in the USA, so you can get fired for anything, including calling in sick, or nothing, no reason at all. Your employer does not have to be fair. You just have to do your job and be grateful for it, or so says Glass Door, an employment guidance site.
Legally speaking, most employment contracts are at-will, meaning you can go and you can be let go for any reason or no reason. But employment is a relationship and you provide a service, so let us not live in fear. Try to be reasonable, and do your job, and you should not get fired for needing time unless your boss is a tyrant.
If you have paid sick days because you are a full-time employee, feel free to take these when you do not feel well enough to work. Theoretically, this should be no problem, but be careful about when you take time. Don't do it right before a big presentation when everyone is expecting you.
"An administrator in my client's department had a key role in planning a major event, and the admin called in sick for three days the week before the event," says Roy Cohen, a New York City career coach and author of The Wall Street Professional's Survival Guide. "All of the work gets dropped on others' shoulders. Her sick days might have been legit, but they were legit too frequently." So make an effort to show up when big things are in the works and you probably won't get fired for taking time.
Interestingly, if you are fired, you may not be given a reason. Corporations make it a policy of saying nothing to avoid liability. Cohen explains, "Assuming you're in an employment-at-will environment, you don't ever have to be told why you're being fired."
For contract workers who have no paid sick days the situation is more complicated. Contractors are free agents, or they are by law. The reality on the ground is different, however. Although contract workers have flexibility about the time of work and location according to federal labor guidelines, employers tend to focus on aspects of the arrangement that favor them. Contractors cover vacation and sick days of employees but get none themselves, no benefits.
If you're in that situation, you're not alone. Still, sick days are the biggest risk for you. You disappoint a boss who is already uncommitted and don't get paid, so it's awkward and costly. The fact remains, however, that people get sick. Contractors, with tentative employment and no benefits are stressed out, which leads to physical illness.
If you have been fired from a job for taking sick days, or any other reason, talk to a lawyer. Tell your story. Many attorneys consult for free or a minimal fee and will be happy to assess your case.