A vaccine uses a live pathogen that is too weak to make you sick. Once it enters your body you defeat it easily. Now that yur body is familiar with it your body can fight off the disease in the future.
Because it creates energy and food for different beings
There doesn't seem to be a maximum number. Ultra high molecular weight polyethylene has around 500 thousand carbons that are linked together.
It is a measure of both species richness and species evenness, or how naturally diverse a specific habitat is. The higher the biodiversity, the larger the ecosystem.
Species richness-number of individuals of a population in a habitat.
Species evenness-number of different populations in a habitat.<span />
