A dream that you have an infection is often a sign that you are being or have been infected with the negative attitudes and thought patterns of other people.
Sometimes, these negative thought patterns can go back to your childhood. Your dreaming mind is letting you know that some behaviors or ideas that you learned from parents or other adults are not healthy for you, and you should try to develop new habits and new ways of looking at the world.
Your dream could also be warning you about negative influences from people you have met more recently.
The part of your body that is infected can play a significant part in the meaning of your dream. For example, if your hand is infected, your dream can mean that you have been made to give up control of certain aspects of your life.
Your physical condition is sometimes reflected in your dreams, so dreams about having an infection can mean that you do have a physical infection in real life.