Fast food workers usually go to work sick as their cheap assed bosses don't like to pay sick pay.
God only knows if one of them coughed, spit-up, vomited, bled, or passed pus from open wounds into your burrito.
God only knows how many millions of viruses, bacteria that burrito you ate may have contained.
Some of that may show up later as any number of diseases that will make you sick.
To be safe, do a google on communicable disease, hepatitis, aids and Ebola virus symptoms to be sure that you are ready for all possible disease attacks.
Having the shits after eating that burrito is just your body's way of telling you that burrito was full of disease causing bacteria or viruses and is the way your body is trying to get it the fuck out of you.
Don't worry about all of that though...