Are all mutations harmful?

No, not all mutations are harmful. Some mutations may be beneficial, while others have no effect on an organism. A change in a DNA sequence that positively affects an organism is commonly referred to as a beneficial mutation. This is often seen in various types of bacteria where mutations lead to antibiotic-resistant strains. In some rare instances, scientists have discovered mutations in humans that have increased their resistance to HIV.  These individuals possess mutations in the receptor proteins of the plasma membrane, which makes it difficult for HIV to bind and infect the host cell correctly.