I've had several spinal taps, when doctors were trying to diagnose various sicknesses I had over the years. None ever showed a positive for any infection.
The worst was in 1988, when I'd been sick for 3 weeks, running a very high fever and becoming delirious several times at home. The Army doctors were treating me for a kidney infection, and had even admitted me into a hospital room, but they finally decided that was wrong, and sent me back down to the basement ER for further diagnosis. I was rather shocked they couldn't do it where I already was.
So I stayed in an ER room for 10 hours, lying half unconscious all by myself on a hard gurney, not even a bed, with no food or attention. Apparently emergency admittances kept taking priority over me, as I'd be periodically told as a nurse would pop in on me and then leave without doing anything.
Finally around 8PM a doctor saw me, who decided to do some chest x-rays and then a spinal tap. From x-ray I was taken into a room with large exposed air conditioning duct work, that rumbled loudly and shook the whole room. I was placed on a gurney that vibrated from the din, and told to do a fetal tuck on my side while they entered my spine.
But I was so dehydrated by then that no fluid came out. I was told to put my thumb in a my mouth like a baby and blow hard, to help force some spinal fluid out. Finally after 30 minutes they got a few drops. Then they left me in that incredibly noisy room.
Around 10 PM a doctor returned, and told me my spinal fluid was normal. But he carried an x-ray, that he put on a light box. "See your 2 lungs?" he asked me. "Yes." "Do you see anything different between them?" "Well, one looks all white, and the other one isn't." "Exactly!" he triumphantly declared. "You have pneumonia!"
Well why the fuck did it take you 3 weeks to figure that out, I wondered to myself. I was given some antibiotics, and sent home around 11 pm. To walk (or rather stagger) the half mile to my quarters, when I thought I should have been admitted at least for the night. After just having had a spinal tap, which normally requires some period of prone rest.
Fortunately I responded to the antibiotics quickly, and by the second day after I felt much better. I had to return to the hospital to get a PT waiver, which they had neglected to give me, otherwise I would have been required to do my usual mile run, pushups and other group calisthenics 3 times a week.
And BTW, I was an Army Major at this time, and this was the medical treatment I received. I can only imagine what enlisted Privates endured.