Week 10

University of Doha for Science & Technology

• College of General Education

Microorganisms and Disease: Disease Processes

• Infectious diseases are caused by organisms that spread from person to person.

• Communicable diseases can spread from person to person.

• Infectious diseases can spread through ingestion, bites, coughs.

• Non-Infectious diseases are not caused by a disease.

• Non-communicable diseases aren't spread.

• They can be caused by genetics, the environment, lifestyle or nutritional issues.

Microorganisms and Disease

• Infectious agents can be viruses, bacteria, protozoans, fungi, and worms.

• The infection process occurs in stages:

  • Exposure
  • Establish focus of infection
  • Colonization and overcoming host defenses

• Surfaces for infection can be skin, gastrointestinal, respiratory, or urogenital tracts.

Human Infectious Diseases

• Bacteria are responsible for various diseases. Examples include Tuberculosis, Pneumonia streptococcal infections, Gonorrhea, Scarlet Fever, Tetanus, Diphtheria, Pertussis, and Bubonic Plague.

• Viruses cause various diseases. Examples include AIDS/HIV, Influenza, Herpes, Hepatitis, Common Cold, Poliomyelitis, Measles, Mumps, and Rubella, Infectious mononucleosis.

• Fungi cause diseases such as Candidiasis, Athlete's foot, Jock itch, Nail fungus, and Ringworm.

• Protozoa cause diseases such as Amebiasis, Amebic meningoencephalitis, Malaria, Trichomoniasis, Toxoplasmosis, Giardiasis, Balantidiasis, Cryptosporidiosis, and Pneumocystosis.

• Helminthes/Parasites include Trichuriasis, Hookworm, Lympathic filariasis, Schistosomiasis, and Ascariasis.

Pathogenicity

• Pathogenicity is the ability of a pathogen to cause disease.

• Some pathogens more easily cause disease than others.

• High pathogenicity examples include Yersinia pestis, Listeria species, and Clostridium tetani.

• Low pathogenicity examples include Staphylococcus aureus.

Virulence

• Virulence is the severity of illness caused by a pathogen.

• All pathogens cause some level of disease.

• High virulence pathogens cause severe illness.

• Low virulence pathogens cause less severe illness.

Examples of Illness caused by different virulence levels

• Common colds generally cause less severe illness.

• H5N1 influenza can cause severe symptoms like pneumonia and multi-organ failure and is more virulent than common influenza.

• COVID-19 and Influenza share similar symptoms:

  • Both start 1-4 days or 1-14 days after exposure.
  • Both have varying degrees of severity.

Sources of Infection: Reservoirs

• Reservoirs are sources of infection where a pathogen can survive until it's passed to a host.

• Can be a living or non-living object.

• Infected humans are a reservoir from consumption of infected food or through transmission of viruses and bacteria.

• Animals can be reservoirs such as monkeys for malaria, cats for diseases, dogs for many diseases (including rabies), and various wildlife.

Human Carriers - Classifications

• A passive carrier never shows outward signs of disease.

• An incubatory carrier can transmit pathogens during the incubation period. Examples include measles, mumps, and polio.

• A convalescent carrier transmits disease while recovering. Examples include cholera, typhoid fever, and diphtheria.

• An active carrier has recovered but can still transmit disease indefinitely.

• Tyhpoid Mary is an example of an asymptomatic carrier.

Non-Living Reservoirs

• Soil can harbor bacteria like Clostridium leading to conditions like tetanus and gangrene.

• Water can harbor Vibrio cholera which causes extreme diarrhea and dehydration.

• Airborne pathogens (e.g., anthrax, chickenpox, influenza, measles, and TB).

Portals of Entry/Exit

• Pathogens need entry portals (e.g., eyes, ears, cuts, abrasions, mouth) to cause infection within the host.

• Disease exit portals (e.g., eyes, ears, mouth, anus, blood) when infectious agents leave the host.

Modes of Transmission

• Transmission of diseases occurs when a reservoir spreads infection.

• This happens from a portal of exit to another organism.

• There are three transmission modes: contact, vehicle, and vector.

Disease Transmission

• There are horizontal and vertical transmission modes.

• Horizontal transmission includes human-to-human, animal-to-human, and sexual contact.

• Vertical transmission includes diseases acquired from mother to fetus like syphilis or gonorrhea, congenital conjunctivitis.

Contact Transmission

• Contact spread occurs when an infectious disease transmits from an infected individual to a new host.

• This transmission can be direct, indirect, or droplet.

• Direct contact is from body contact. Examples include shaking hands, kissing, or sexual contact.

• Indirect contact is from a contact with non-living objects (e.g., doorknobs, fomites).

• Droplet spread is from respiratory droplets released by an infected individual.

Vehicle Transmission

• Vehicle transmission occurs when a non-living object spreads infection.

• Examples include waterborne (cholera), airborne (flu), and foodborne (salmonella).

Vector Transmission

• Vector transmission involves a living organism (vector) that carries pathogens from a reservoir to a host.

• Examples include mosquitoes (malaria), fleas (bubonic plague), lice, flies (African Sleeping Sickness).

Nosocomial Transmission

• Nosocomial infections are infections acquired in healthcare facilities. These can be secondary to a pre-existing condition or occur shortly after admission/within 14 days of discharge.

• Iatrogenic infections result from medical procedures, surgeries or healthcare staff.

Bacteria causing Nosocomial Infections

• Staphylococcus aureus (frequently resistant to antibiotics, like MRSA) is a common example.

• Other examples include Escherichia coli, Enterococci, and Candida. Infectious microorganisms are often found on skin and mucous membranes.

How Do Bacteria Cause Disease?

• Virulence factors are structural/physiological characteristics of bacteria that allow them to cause infection.

• Adherence characteristics of microbes allow bacteria to stick to host cells, a key initial step to infection.

• Evasion helps pathogens escape the host's immune defenses.

• Toxins damage host cells/tissues.

Bacterial Adherence & Infection

• Adhesins are molecules bacteria use to attach to host cells.

• Capsules, pili/fimbriae are examples of adherence factors that contribute to bacterial attachment and infection.

Invasiveness/Evasion Mechanisms

• Invasiveness helps bacteria spread through tissues (e.g., hyaluronidase enzyme).

• Evasiveness mechanisms allow bacteria to avoid immune responses (e.g., coagulase enzyme, streptokinase enzyme).

Bacterial Toxins

• Endotoxins are part of the cell wall; release causes fever, shock.

• Exotoxins are secreted proteins; cause more potent, damaging effects (hemolysins, leukocidins, responsible for diseases like botulism, tetanus, cholera).