Answer:
Because 2 forces are stopping the crate, friction and gravity. While only one force is trying to move it forward.
A vaccine is a preparation of one or more microbial antigens to induce protective immunity in potential hosts.
Vaccines are the anti-microbial agents administered to provide immunity against specific diseases. There can be a wide variety of vaccines, like: live attenuated vaccine, vaccine with killed or inactivated microbes, vaccine containing the sub-parts of the microbes, etc.
Immunity is the body's ability fight against a disease. The immunity is build when a body gets attacked by pathogen. Various immune cells like neutrophils, MHCs, dendritic cells, B and T cells, etc. are involved in building and strengthening the immunity of any person.
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