• arthropod feeds on skin of person or animal who has Rickettsiae
• arthropod becomes infected and deposits disease in skin of healthy person
• person scratching infection site, forcing Rickettsiae into tissue
• person becomes ill

round (coccus-shaped)

Gram-negative cells

• interpersonal contact
• does not require an insect or tick vector for transmission

• elementary body (EB) - infectious stage
• reticulate body (RB) - reproductive stage once inside the infected cell

• Nongonococcal urethritis
• Trachoma blindness

• most common sexually transmitted disease
• inflammation of the urethra in females can lead to PID or pelvic inflammatory disease

disease that occurs at irregular intervals 

ex. rabies

disease found at a low but steady level of frequency

ex. colds, STDs

disease occurrence that is above expected levels

ex. a bad flu season 

a sudden (usually localized) increase in disease

ex. food poisoning

disease occurrence over a large population (ie. whole continents, entire world)

ex. AIDS

• slow rise, low peak, slow decline
• multiple incubation periods
• ex. influenza, chicken pox, measles

• fast rise, high peak, fast decline
• 1 or 2 incubation periods
• ex. food poisoning, legionella

• passed from person to person
• all communicable diseases are infectious

• easily passed from person to person
• all contagious diseases are communicable

Infectious, communicable, or contagious?

influenza

Infectious, communicable, or contagious?

gonorrhea

Infectious, communicable, or contagious?

tetanus

Infectious, communicable, or contagious?

HIV

Infectious, communicable, or contagious?

Lyme disease

Infectious, communicable, or contagious?

common cold

• Organism
• Source or Reservoir
• Transmission
• Host Susceptibility
• Exit Mechanism

may be:

• human
• animal (zoonosis)
• non-living

may be by:

• airborne: more than 1 meter
• contact: less than 1 meter
• vehicle: non-living method (also called fomite)
• vector: living method
  • harborage vector
  • biologic vector

pathogen carried inside of organism ( may be where it develops) (also called an internal vector)

ex. mosquitoes, ticks 

takes into account two factors

• immune system of the host
• pathogenicity of the invading organism

creates 4 possible situations

Host Susceptibility:

strong immune system of the host + weak pathogenicity of the organism =

Host Susceptibility:

strong immune system of the host + strong pathogenicity of the organism =

Host Susceptibility: 

weak immune system of the host + strong pathogenicity of the organism =

Host Susceptibility:

weak immune system of host + weak pathogenicity of the organism  =

• flies
• fingers
• food
• feces
• fomites (non-living transmission)

• respiratory system
• digestive system
• genitourinary system
• skin (wound/sores) system

• reduce the source of infection (breaking the chain at the source or reservoir stage)
• break the connection between the source of the infection and the susceptible hosts (breaking the chain at the transmission stage)
• eliminating susceptible individuals (targeted at the host susceptibility stage)

• isolation (ex. MRSA patients)
• individual therapy - makes a person less infective to others (ex. antibiotics for infections or decongestant for colds)

• destruction of vehicles (ex. pasteurization of milk and chlorination of water)
• destruction of non-human vectors (ex. spraying to kill mosquitoes in the summer

no symptoms of disease (asymptomatic or subclinical) but can still infect others 

ex. Typhoid Mary

does not get disease, but passes disease from infected to uninfected 

ex. poor sterile technique

number of organisms that must enter host for a disease to become established (most diseases require more than one organism in a dose)

ex. 10 virons for rabies; 1000 cells for gonorrhea; 10,000 cells for salmonellosis; 1,000,000 cells for cholera; a smaller amount of late stage HIV blood, larger amount of early stage HIV blood

• toxic enzymes that contribute to disease process by helping the parasite overcome body defenses
• ex.
  • coagulase
  • streptokinase
  • hemolysins
  • leukocydins
  • exfoliative toxins
  • enterotoxins

• interfere with nerve/muscle communication (tetanus)
• interrupt protein synthesis in the cell (diphtheria)
• decrease blood pressure (hypotension or shock)
• induce vomiting/diarrhea/dehydration
• increase body temperature (fever)

toxoids - an inactivated version of the exotoxin injected into the body to stimulate the immune system to produce anti-toxins

ex. the D and T in DTaP

deadly substances known

ex. the amount of pure botulism toxin that would be contained on the head of a pin is lethal to a human

• the A subunit is the toxic portion
• the B subunit is not toxic, but is required for entry of the A subunit into a cell
• both are required for toxic effects

• diphtheria
• pertussis
• tetanus
• botulism
• gas gangrene

• death of the cell, not as a normal byproduct of metabolism
• some illnesses caused by Gram - organisms are not treated with ATBs as they may make illness worse
• Ex. salmonella

• salmonellosis
• dysenteries
• fevers

• produced by Gram +
• released while cell is still living
• vaccinated against
• very deadly
• ex.
  • diphtheria
  • pertussis
  • tetanus
  • botulism
  • gas gangrene

• produced by Gram -
• released when cell dies
• not easily vaccinated against
• not as severe
• ex.
  • salmonellosis
  • dysenteries
  • fevers