This review provides an overview of vaccines, immunization, and related issues, aiming to inform a broad scientific audience about the underlying immunological concepts. Vaccines are biological products that induce immune responses to confer protection against infection and disease. They contain antigens derived from pathogens or synthesized to represent pathogen components. The essential component of most vaccines is one or more protein antigens that induce immune responses providing protection. Polysaccharide antigens can also induce protective immune responses and are used in vaccines against bacterial infections like pneumonia and meningitis. Vaccines exploit the human immune system's ability to respond to and remember pathogen antigens. However, vaccines have historically been developed empirically without immunologists' involvement. There is a need for improved understanding of immunological principles to develop vaccines for hard-to-target pathogens, control outbreaks, and revive immune responses in the aging population. The review discusses the history of vaccination, vaccine development, immunological basis, and impact on infectious diseases. It also highlights current and future challenges in protecting the world from common pathogens and emerging infectious threats. Vaccines are classified as live or non-live, with live vaccines containing attenuated strains of pathogens and non-live vaccines containing pathogen components or killed organisms. Live vaccines may replicate in immunocompromised individuals, leading to restrictions in their use. Non-live vaccines are generally combined with adjuvants to enhance immune responses. Adjuvants like AS01 and AS04 stimulate the innate immune system and enhance adaptive immune responses. Vaccines induce antibodies, which are crucial for protection. However, T cell help is also important, especially for certain vaccines. The review discusses the role of T cells in vaccine-induced protection and the importance of immune memory. Vaccine-induced immune memory can provide protection against future infections, but it may not be sufficient for rapidly invasive bacterial infections. Booster doses are often necessary to maintain immunity. Herd immunity is an important feature of vaccine-induced protection, as it reduces the spread of pathogens within a population. Vaccines can prevent infection or disease, and some vaccines, like the BCG vaccine, can prevent both infection and severe disease. Non-specific effects of vaccination, such as changes in immune responsiveness, can also provide protection against unrelated pathogens. Factors affecting vaccine protection include age, maternal antibody levels, prior antigen exposure, vaccine schedule, and dose. Older adults often have limited protection from vaccines due to immunosenescence, the decline in immune function with age. New vaccine technologies, such as high-dose or adjuvanted vaccines, may help overcome immunosenescence and improve protection in older adults. The review emphasizes the importance of understanding immunological mechanisms to improve vaccine development and public health outcomes.This review provides an overview of vaccines, immunization, and related issues, aiming to inform a broad scientific audience about the underlying immunological concepts. Vaccines are biological products that induce immune responses to confer protection against infection and disease. They contain antigens derived from pathogens or synthesized to represent pathogen components. The essential component of most vaccines is one or more protein antigens that induce immune responses providing protection. Polysaccharide antigens can also induce protective immune responses and are used in vaccines against bacterial infections like pneumonia and meningitis. Vaccines exploit the human immune system's ability to respond to and remember pathogen antigens. However, vaccines have historically been developed empirically without immunologists' involvement. There is a need for improved understanding of immunological principles to develop vaccines for hard-to-target pathogens, control outbreaks, and revive immune responses in the aging population. The review discusses the history of vaccination, vaccine development, immunological basis, and impact on infectious diseases. It also highlights current and future challenges in protecting the world from common pathogens and emerging infectious threats. Vaccines are classified as live or non-live, with live vaccines containing attenuated strains of pathogens and non-live vaccines containing pathogen components or killed organisms. Live vaccines may replicate in immunocompromised individuals, leading to restrictions in their use. Non-live vaccines are generally combined with adjuvants to enhance immune responses. Adjuvants like AS01 and AS04 stimulate the innate immune system and enhance adaptive immune responses. Vaccines induce antibodies, which are crucial for protection. However, T cell help is also important, especially for certain vaccines. The review discusses the role of T cells in vaccine-induced protection and the importance of immune memory. Vaccine-induced immune memory can provide protection against future infections, but it may not be sufficient for rapidly invasive bacterial infections. Booster doses are often necessary to maintain immunity. Herd immunity is an important feature of vaccine-induced protection, as it reduces the spread of pathogens within a population. Vaccines can prevent infection or disease, and some vaccines, like the BCG vaccine, can prevent both infection and severe disease. Non-specific effects of vaccination, such as changes in immune responsiveness, can also provide protection against unrelated pathogens. Factors affecting vaccine protection include age, maternal antibody levels, prior antigen exposure, vaccine schedule, and dose. Older adults often have limited protection from vaccines due to immunosenescence, the decline in immune function with age. New vaccine technologies, such as high-dose or adjuvanted vaccines, may help overcome immunosenescence and improve protection in older adults. The review emphasizes the importance of understanding immunological mechanisms to improve vaccine development and public health outcomes.