Bacteria are living cells and when they invade a host, they can cause illness or death, if they get past the immune system. Antibiotics can work on bacterial cells by affecting their ability to turn glucose into energy, which is the process of cellular respiration. Some can prevent their ability to construct their cell wall. A bacteriocidal kills the bacteria while a bacteriostatic stops them from multiplying. Some antibiotics prevent protein synthesis and the bacteria die. Some prevent them from manufacturing folic acid, a necessary substance for their survival and some interfere with the production of the peptidoglycan molecule in the cell wall, resulting in fragile cell walls and cell death.