Disease causing organisms can be classified as Obligate, Accidental or Opportunistic. Obligate Pathogens are organisms that cannot survive for long outside of the human body. An example that we will discuss later is Neisseria gonorrhoeae , the causal agent of gonorrhea. Opportunistic Pathogens are organisms that can only cause disease in an immuno-suppressed person or under unusual circumstances, e.g. skin infections caused by Pseudomonas aeruginosa when the normal skin microbiota is disrupted. We also will be referring to the degree of virulence of different pathogens.
Virulence is a relative term, a highly virulent organism is one that can cause an acute infection.
Vector-borne e.g. via ticks, fleas etc. Before a pathogen can cause infection it must attach to a specific tissue. Most nasty diseases start when organisms adhere to mucosal surfaces inside the respiratory, gastrointestinal or genitourinary tracts. Most pathogens have very specific mechanism for attachment to the specific mucosal surfaces that they colonize. For example E. coli strains that attach to glycoproteins on mucosal cells in the urinary tract have different lectins on their fimbriae than E. coli that cause intestinal infections.