Ok. so in the average drop of blood there are around 20000 white blood cells. They just float around.
Then, during infection: an unknown bacterium enters the body. The white blood cells all rush to the bacterium and destroy it through a process known as phagotocysis(they swallow the bacterium), and destructive enzymes within the WBC (white blood cell), will destroy the bacterium.
However, what if the infection fights BACK? These are your everday colds, viruses, flu, etc. The infection fights back: causing stuffy nose, fever, etc. Then, your WBC count rises DRAMATICALLY. Usually the number doubles (goes to 40,000). Now the guy below me said that they don't, and just antibodies increase. Antibodies do increase, but they are produced by the WBC which ALSO increase.
The only problem is: when you have constant high numbers of WBC at a count of around 50,000 in each blood drop: then you have leukemia which is a deadly cancer.
So in short words: Yes, during infection the WBC have increased. Temporarily though, AFTER infection they will go back to normal.
Khanacademy, Human Body Textbook, Google, Wikipedia, Wiki answers, Youtube.