Pathogen must be able to bind to some receptor molecule on cell surfaces. These typically have necessary functions for cell. Most diseases are tissue specific, because only certain tissues have receptor molecule needed. Fimbriae or pili are used by some bacteria to attach selectively to certain tissues. Tricks to avoid host defenses .
Hydrolytic enzymes can destroy most organic molecules.
Capsules Many pathogens have thick extracellular polysaccharide capsules. Capsules inhibit phagocytosis, prevent quick disposal of bacterium by WBCs. Loss of capsule typically causes loss of infectivity. Accumulation of pus at infected site is caused by dead WBCs. WBCs, other body defenses can't reach site of infection.