What diseases cause high fevers and high white blood cell count?

Usually bacterial. It's usually a bacterial illness that causes the symptoms you describe. But this can include anything form sinus infections, tonsillitis, ear infections, pneumonia, infection in the blood, urinary tract infections, appendicitis, meningitis, skin abcesses... The list goes on and on. You need a thorough assessment of associated symptoms and a physical exam to try and pinpoint your condition.