I'm always sick and idk what to do

We think I have an immune system deficiency but I still need to get tested for it and that costs a lot of money that I don't have. Iv been working as a cashier for about 6 months (it's my first job and I'm 21) and I keep getting really sick and having to call off. At first I was going to urgent care when I got sick to get medicine and a doctors note so I wouldn't get fired, but urgent care is expensive and I don't have the money to keep going. I woke up at 4am this morning with a fever and cough and sinus pain so I shouldn't go into work because I can get others sick but I don't want to get fired. What should I do?