Subunit vaccines are a type of vaccine that contain only specific pieces of a virus or bacterium, rather than the whole pathogen. These pieces, often proteins or sugars, are enough to stimulate an immune response without causing disease. By focusing on these subunits, the vaccine can effectively train the immune system to recognize and fight the actual pathogen if encountered in the future. One common example of a subunit vaccine is the Hepatitis B vaccine, which uses a protein from the virus to elicit immunity. Subunit vaccines are generally considered safe, as they do not contain live pathogens, reducing the risk of infection. They can be combined with adjuvants to enhance the immune response, making them effective in preventing various infectious diseases.