Cat has a low white blood cell count?

About a month ago my cat went to the emergency vet because he was really lethargic and was only breathing through his mouth.  We got X-rays and a blood test done, and the blood test showed a low white blood cell count. The vet ended up keeping him overnight on fluids and gave us antibiotics.  They said this is to help kill any secondary bacterial infection, since this was believed to be viral.  They thought that whatever was going on with him was upper respiratory, but they didn't know what the specific problem was.  They needed an expensive cat-scan, which we declined.  We gave him the full dose of antibiotics, and to this day he is active, eating, drinking.  I just came across the file while cleaning and am wondering what could have caused this low of white blood cells.

Blood test results for the low ones:

WBC                        2.55 K/uL

NEU                         1.41 K/uL

I can post a picture of the blood results sheet if anyone needs.

Thanks.

There is a lot more that I didn't put in there, so if anyone needs I can put the rest in, but it is quite a bit.

It's really hard to say what bug your kitty had, I am glad he is well now.  I do know that with humans a very low white cell count is indicative of a virus.  I took my oldest one to the doctor when she was really sick.  They took a blood sample and it came back with a ridiculously low white cell count and of course with my first child and she was still a toddler I freaked out.  The doctor told me that it was a particularly nasty virus and it would just have to run it's course.  It does explain why a virus often turns into some type of bacterial infection.

Thanks for your reply Denice. I just need some reassurance tha this will go away over time, because he isn't completely recovered yet.