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WBC or white blood cells increase as a result of infection. It is the white blood cells that fight off infection so when you get an infection the count of wbc increases as they go to work to fight off the infection.

White blood cells, or leukocytes, are cells of the immune system defending the body against both infectious disease and foreign materials. Several different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a hematopoietic stem cell. Leukocytes are found throughout the body, including the blood and lymphatic system.[1]

The number of leukocytes in the blood is often an indicator of disease. There are normally between 4?109 and 11?109 white blood cells in a litre of blood, making up approximately 1% of blood in a healthy adult.[2] In conditions such as leukemia, the number of leukocytes is higher than normal, and in leukopenia, this number is much lower. The physical properties of leukocytes, such as volume, conductivity, and granularity, may change due to activation, the presence of immature cells, or the presence of malignant leukocytes in leukemia.

http://en.wikipedia.org/wiki/White_blood_cell