“Do Boond zindagi ki” When Amitabh Bachchan voiced those words, millions of parents in India rushed their kids below the age of five to the nearest polio booth to get them vaccinated. The outcome? In 2016, WHO declared India a “polio-free nation” marking its victory over a twenty year long battle against the crippling disease of poliomyelitis.Before the advent of vaccines, a person could become immune to a disease only by getting the disease and with luck, surviving it. This way of naturally acquired resistance required one to suffer all the symptoms of an infection, risk the complications, which could be fatal, and isolate oneself from society to keep a contagious disease from spreading. Vaccines provide a safer alternative of artificially imparting active immunity to a person without actually causing the disease or risking its transmission to healthy individuals.Today, diseases like plague, polio, tetanus, typhoid, whooping cough that were once prevalent are all but forgotten in most parts of the world and much decreased in occurence in other regions. This achievement in worldwide health has been possible only by the emergence of vaccination; proven to be the best method of disease prevention in both safety and cost efficiency.Vaccination is a means of educating the immune system to recognize and fight a pathogen effectively, protecting the body from the infectious disease. It is based on the ability of our immune system to distinguish between self and non self, and elicit an immune response against the foreign agent, introduced through vaccine, which it remembers for a long time. So the next exposure to the same foreign particle triggers a much stronger immune response that kills the pathogen upon entry and no ailment ensues. Edward Jenner, an English physician, is mostly credited for the development of vaccination through the inoculation of an eight year old boy with fluid from a cow pox postule of a milkmaid, resulting in successful immunity against smallpox in 1796. Although Jenner’s contribution was significant, it was only one of many.About twenty years before Jenner, a farmer in England named Benjamin Jesty innoculated his family with cowpox virus to induce resistance to smallpox. He exposed his children to smallpox patients to prove their immunity.However, as it has been stated by scientist Francis Galton,”In science credit goes to the man who convinces the world, not the man to whom the idea first occurs”. Jenner was able to publish his work and propagate the idea of vaccination strongly, becoming the pioneer of vaccination technique.The first attempts to induce immunity date back to the fifteenth century, when the Chinese practised nasal insufflation wherein dried crusts from smallpox pustules were inhaled up the nostrils to immunize healthy individuals. Smallpox inoculation or variolation was also practised in Turkey and parts of Africa, in which fluid from pustules was rubbed into cuts made in the skin of a healthy person rendering him immune to smallpox.During the “milder disease” outbreaks in seventeenth century India, pox-laden blankets were collected and susceptible children were wrapped in them with the intention of transmitting mild disease.In the eighteenth century, variolation got introduced to Europe when Reverend Cotton Mather learnt about it from African slaves and began to promote the practice from there on. Also in 1718, Lady Mary Wortley Montagu, wife of ambassador to Turkey, heard about it on arrival in Turkey and got her children variolated.In 1880, Louis Pasteur presented his finding that an old culture of cholera bacteria administered to chickens rendered them protected against cholera. He coined the term vaccine ( _vacca_ means “cow” in Latin), in honour of Jenner’s work, for the attenuated strain he used. He extended the discovery to other pathogens and came up with vaccines for anthrax and rabies. The development of vaccination led to the study and understanding of the immune system, laying the foundation of immunology as a new field of medical science.Twentieth century saw the emergence of a number of vaccines against various diseases. Different types of vaccines exist in use today:1. Live attenuated vaccines contain active but weakened microorganisms that are non-virulent.2. Inactivated vaccines contain pathogens killed by chemicals or heat.3. Toxoid vaccines consist of inactivated exotoxins produced by the organism.4. Subunit vaccines contain a protein subunit of the pathogen.5. Conjugate vaccines have a capsular polysaccharide linked to a protein carrier.DNA vaccines and recombinant vector vaccines that can deliver antigen encoding genes and directly activate immune cells are under clinical trials.The main aim of vaccination is to stimulate the adaptive immune system without provoking an acute inflammatory response of the innate immune system which would result in symptoms of the disease. Following injection, the antigens in the vaccine attract dendritic cells, monocytes and neutrophils that circulate throughout the body. Elicitation of sufficient signals by the antigens activates the helper T lymphocytes and cytotoxic T lymphocytes which mediate killing of altered self cells. They further generate B lymphocytes which mature into plasma cells and secrete antibodies that bind to and clear the antigens from the body. Some activated T and B cells become memory cells that remain in the body and help the system remember the antigen until the next infection to trigger a more potent response in less time. Nowadays, vaccination of children is begun at birth and a schedule recommended by WHO is followed upto 18 years of age. International travellers are also routinely vaccinated for diseases endemic to their destinations.A milestone in vaccinology was witnessed with the elimination of small pox from the world in 1977. While the success of a similar drive against polio is still awaited, vaccination is posed by challenges of resistance, in certain areas, based on rumours of association of vaccines with sterility, autism. Though unfounded, these misconceptions need to be overcome for successful immunization of the masses.Vaccination saves 6 million lives every year. Vaccination is not about protecting one person, it’s about protecting the entire community- it is public health in the truest sense.The success of smallpox eradication has made worldwide immunization to various diseases a realistic proposal. All we require is to develop better, safer, cheaper, easier to administer vaccines.