Sporulation occurs under an unfavorable environment like high temperature, ph., etc. Where a normal cell cannot survive. So bacteria and fungi can convert their cells into a spore. These spores have all the nutrition required for growth but have very less water content which makes it hard and unreactive to the harsh environment. Also, some spores have an extra protective covering of lipids, polysaccharides or some proteinaceous substance that makes it withstand the harsh environment.