There's a difference between infection and disease. Infection, often the first step in getting a disease, occurs when bacteria, viruses or other microbes that cause disease enter the body and begin to multiply. Disease happens when the infection damages cells in the body. Then symptoms of an illness appear.
In response to infection, the immune system becomes active. White blood cells, antibodies and more goes to work to rid the body of what's causing the infection. For instance, in clearing out the common cold, the body might react with fever, coughing and sneezing.