The nature of pathogens

Bacteria
Some yeast, Candida is commonly present
A few protozoa
Latent viruses like Epstein-Barr virus (glandular fever)
Herpes Simplex (cold sores)

organisms that are removed quickly, eg by washing

They create an environment which will reduce colonisation by other species eg reduce the pH
Produce antibacterial chemicals
Intestinal bacteria produce vitamins used by the host
Prevent infection eg pseudomembranous colitis, thrush which can occur when the normal flora are removed by antibiotic therapy

infection by pathogen from the human host, eg skin bacterium like Staphylococcus epidermidis, or Staph aureus from the nose causing wound infection, eg E. Coli from the colon causing bladder infection

a type of endogenous infection , a pathogen is transferred to foetus or neo-nate from its mother eg rubella, syphilis toxoplasmosis, hepatitis group B streptococci, HIV

infection by organism from external environment eg another person

infection acquired during a patient’s stay in hospital.
Previously called Hospital acquired Infection (HAI) and
Nosocomial Infections

infection by a microorganism that would not normally cause infection, but do so because of alteration in the condition of the host, or displacement of the microorganism.

the microorganism is ACQUIRED by the host
COLONISATION occurs
host tissues are PENETRATED
the microorganism SPREADS within host tissues
the microorganism DAMAGES host tissues
the infection is RESOLVED by host defences

Microorganisms acquired from exogenous sources need a Route of Transmission. The major ones are;
Direct (horizontal, vertical, animal)
Indirect (fomite, faecal – oral (hands, food), droplet (cough, sneeze, talk) vehicle (food, air, water), vector (mechanical eg fly, biological eg mosquito

refers to the presence and growth of a microorganism on skin or mucus membranes without evidence of infection.
A common scenario is for a patient admitted to hospital to be colonised with hospital strains

The ability to cause disease

Differences in pathogenicity are also referred to as virulence of the organism

Infectivity - how easily the microbe enters the host, attaches to host cells or tissues, survives the normal host defenses and establishes infection
The severity of the damage it causes the infected host.

the number of microorganisms required to cause observable infectious disease

Factors which favour the establishment of Disease