Protein subunit vaccines are a type of vaccine that use harmless pieces of the target pathogen, often proteins, to stimulate an immune response. Unlike traditional vaccines that use live or inactivated viruses, these vaccines focus on specific proteins that are essential for the pathogen's ability to cause disease. This approach helps the immune system recognize and fight the actual pathogen if exposed in the future. These vaccines are generally considered safe because they do not contain live components of the virus, reducing the risk of infection. Examples of protein subunit vaccines include those developed for diseases like Hepatitis B and Human Papillomavirus (HPV).