Infection by bacteria is successful invasion on the host tissue and such bacteria causing infection are called pathogens. There are natural pathogens or true pathogens which have got the inherent capacity to invade a normal tissue also whereas an opportunistic pathogen can cause infection only when the host defense mechanisms are altered or lowered. The successful invasion on the tissue by bacteria depends upon the virulence of the bacteria. Many bacteria secrete enzymes like hyaluronidase, lecithinase, lipases collagenase, coagulase, which act on the host tissue and cause dissolution of the basement membrane of the cells or phospholipids of the cells or the connective tissue of the host and such damage to the tissue render the bacteria to invade the host more easily.