Cells are the basic unit of all oranisms, they contain almost everything we need to survive. They form our organs and blood, etc. That are the building blocks for all life, whether the organism is single-cellular or multi-cellular.
Because if they don’t control infections on one group of people the infection could spread out to large numbers of people causing big numbers to be infected
During the second trimester of pregnancy you can expect your baby’s or babies bones to begin to harden.