A pathogen is a microorganism that can cause disease. There are five main types of pathogenic organisms: viruses, bacteria, fungi, protozoa, and worms. Viruses and bacteria are the most common pathogens, and include organisms such as influenza, HIV, E. coli, and Salmonella that infect humans through airborne transmission, bodily fluids, food/water, or insect bites.

1. What is a
pathogen?
INFECTION PREVENTION
AND CONTROL

2. What is a
pathogen?
A pathogen is a bacterium, virus, or other
microorganism that can cause disease.
In biology, a pathogen, in the oldest and broadest sense,
is anything that can produce disease. A pathogen may
also be referred to as an infectious agent, or simply a
germ. The term pathogen came into use in the 1880s.
A variety of microorganisms can cause disease.
Pathogenic organisms are of five main types: viruses,
bacteria, fungi, protozoa, and worms.

3. Virus
 DNA Virus
 Uses DNA as its genetic material.
 Replicates using DNA-dependent DNA-
polymerase.
 Hepatitis B Virus (some classifications)
 Chickenpox virus (varicella)
 Human papilloma virus
 Herpes virus
 RNA Virus
 Uses RNA as its genetic material.
 Sometimes called riboviruses
 Ebola, SARS, common cold, West Nile,
rabies, influenza, polio
 Subset uses DNA Intermediates for
replication, called retroviruses
 HIV-1
 HIV-2
Human Immunodeficiency Virus
(green)
Coming from host lymphocyte.

4. Bacteria
Bacterial Type: Cocci
 Gram Positive
 Neisseria
 Neisseria gonorrhonae
 Gonorrhea (sexually transmitted)
 Neisseria meningitides
 Many bacterial meningitis cases
Bacterial Type: Cocci
 Gram Negative
 Staphylococci
 Staphylococcus aureus
 Streptococci
 Streptococcus pyrogens
 Strep throat
 Scarlet fever
Cocci
Spherical in chains

5. Bacteria
Bacterial Type: Bacillus
 Gram Positive
 Bacillus anthracis
 Anthrax
 Listeria monocytogenes
 Listeriosis (often meningitis, encephalitis)
Bacterial Type: Bacillus
 Gram Negative
 Salmonella species
 Salmonella gastritis
 Vibrio species
 Vibrio cholerae
 Cholera
 Legionella species
 Legionnaires disease
Bacillus bacteria
(rod shaped)

6. Bacteria • Anaerobic Bacteria
• Does not grow or survive in the
presence of oxygen.
• Grows in oxygen poor or oxygen free
environment.
• Clostridium botulinum
• Botulism
• Clostridium tetani
• Tetanus
• Clostridium perfringens
• Gas gangrene
Patient with gas gangrene of upper left leg prior to amputation.
Source: https://upload.wikimedia.org/wikipedia/commons/8/81/Gas_gangrene.jpg

7. Bacteria • Spirochete Bacteria
• Characteristic spiral shape of
bacteria.
• Varied subspecies, may are
pathogens.
• Treponema pallidum
• Syphilis
• Borrelia burgdorferi
• Lyme disease
• Leptospira interrogans
• Leptospirosis
Treponema pallidum - note spiral shape of bacteria

8. Bacteria • Mycobacterium
• “myco” is the Greek prefix meaning
fungus. This reflects the way that
the bacteria grow in culture,
resembling fungus.
• Mycobacterium tuberculosis
• Tuberculosis
• Mycobacterium leprae
• Hanson’s disease (leprosy)
• Mycobacterium ulcerans
• Buruli ulcers
Micrograph of Mycobacterium tuberculosis

9. Bacteria • Rickettsia
• Gram negative, non-spore forming,
usually cocci shaped obligate
intracellular parasitic bacteria.
• Rickettsia rickettsii
• Rocky Mountain spotted fever
• Rickettsia typhi
• Murine typhus
• Rickettsia prowazekii
• Epidemic typhus
Micrograph of Rickettsia ricketsii living inside cells of a tick.
NOTE: Rickettsia IS NOT associated with the disease “Rickets”.

10. Bacteria • Chlamydia
• Gram negative, obligate intracellular
parasitic bacteria, smaller than
many virus.
• Chlamydia trachomatis
• STD chlamydia, trachoma
(blindness)
• Chlamydophilia pneumoniae
• Form of pneumonia
• Chlamydophilia psattici
• psittacosis (parrot fever)
Micrograph of Chlamydia trachomatis.

11. Bacteria • Mycoplasmas
• Smallest bacteria yet identified.
Often antibiotic resistant due to lack
of a cellular wall.
• Mycoplasma pneumoniae
• Atypical pneumonia
• Mycoplasma genitalium
• Pelvic inflammatory disease
• Mycoplasma incognitus
• suspected in chronic fatigue syndrome
and rheumatoid arthritis
Micrograph of Micoplasma haemofelis
(Feline Infectious Anemia).

12. Fungi • Primarily yeasts and molds.
• Family includes mushrooms. All
species have chitin in their cell walls
making them resistant to toxins and
antibiotics.
• Candida albicans
• Candidias
• Thrush if oral
• Yeast infection if vaginal
• Cryptococcus neoformans
• Fungal meningitis and encephalitis
• Histoplasma capsulatum
• Histoplasmosis (pneumonia from bird droppings)
Candidias (oral thrush) on the tongue of a young woman.

13. Protozoa • Single celled, animal like, microorganisms.
• May be predatory, free-living or parasitic.
• Entamoeba histolytica
• Amoebiasis (amoebic dysentery)
• Toxoplasma gondii
• Toxoplasmosis (usually from exposure to cat feces).
• If infected during pregnancy causes Congenital
Toxoplasmosis in fetus.
• Cryptosporidium
• Cryptosporidiosis (protozoan parasitic bowel disease)
Entamoeba histolytica with ingested human red blood cells (dark circles).

14. Worms • Parasitic worms entering the body by
ingestion or skin penetration.
• Cestodes (multiple species)
• Tape worms (pork, beef or other source)
• Trematodes (flukes, multiple species)
• Liver infection (snail liver) and swimmer’s itch (skin
infection)
• Nematodes (round worms, multiple species)
• Primarily intestinal or intestinal at some phase in life
cycle.
Hook worms attached to intestinal mucosa.

15. Summary
 A pathogen is an organism that causes
disease by infection of a human host.
 Compare to a symbiotic microorganism
that benefits the human host by living in
or on them and benefiting their
physiology (symbiotic coliform bacteria
in the intestinal tract or the flora of the
skin).
 The major types of pathogens are:
 Virus
 Bacteria
 Fungi
 Protozoa
 Worms