Human Diseases: A Systemic Approach

Objectives of the Chapter:
- Define infectious disease and its associated terminology.
- Explain the various methods of transmission for infectious diseases.
- Describe and compare the characteristics of major pathogen categories:
  - Prions
  - Viruses
  - Bacteria
  - Protozoa
  - Fungi
  - Helminths
- Define and discuss nosocomial infections (hospital-acquired infections).
- Explain treatments for bacterial, viral, fungal, and parasitic infectious diseases.
- Understand appropriate antibiotic use and discuss the issue of antibiotic resistance.
- Describe causes and examples of emerging and reemerging infectious diseases.
- Identify common childhood vaccine-preventable infectious diseases.

Infectious Disease: A disease caused by pathogenic microorganisms, such as bacteria, viruses, parasites, or fungi.
Pathogen: Any bacteria, virus, or other microorganism that can cause disease.
Contagious: A term used to describe diseases that are easily spread from one person to another.
Communicable: A term used for diseases that can be transmitted from one individual to another.
Epidemiology: The study of how diseases affect the health and illness of populations.
- Incidence: The number of new cases of a disease during a specified period.
- Prevalence: The total number of cases of a disease existing in a population at a specific time.
Endemic: A disease or condition regularly found and consistently present in a particular geographic area.
Epidemic: An outbreak of a disease that occurs in a larger number of people than expected in a given area over a particular period.
Pandemic: An epidemic that has spread across multiple countries or continents, affecting a large number of people.
Outbreak: A sudden rise in the number of cases of a disease.
Notifiable Diseases: Diseases that must be reported to government authorities when diagnosed.

Reservoir: The natural habitat or host where a pathogen normally lives and multiplies.
Horizontal Transmission: Refers to the direct or indirect transmission of pathogens between individuals of the same generation.
- Direct Transmission: The immediate transfer of a pathogen from one individual to another (e.g., through physical contact).
- Indirect Transmission: Transmission through an intermediate object or organism (e.g., fomites).
Fomites: Inanimate objects that can carry and transmit pathogens (e.g., doorknobs, utensils).
Vertical Transmission: The transmission of pathogens from an infected mother to her offspring during pregnancy or childbirth.
Parenteral Transmission: Referring to routes other than the alimentary canal, where pathogens enter the body through injury or injection.

Nosocomial Infections: Infections acquired in a hospital or healthcare facility that were not present at the time of admission.
Normal Flora: The microorganisms that typically reside in or on the body and play a role in defending against pathogenic organisms.
Microbiota: The community of microorganisms that inhabit a particular environment, including the human body.
Handwashing: A critical practice in preventing nosocomial infections and controlling the spread of pathogens.

Isolation: Separating sick individuals from healthy individuals to prevent the spread of disease.
Quarantine: Restricting the movement of individuals who have been exposed to an infectious disease to prevent transmission.
Disinfection: The process of cleaning something to remove or destroy pathogens.
Bloodborne Pathogens: Pathogens that are transmitted through blood and other bodily fluids.
- OSHA: Occupational Safety and Health Administration, which establishes standards for workplace safety concerning bloodborne pathogens.
- HIV: Human Immunodeficiency Virus, which can lead to AIDS and is transmitted through blood.
- HBV: Hepatitis B Virus, a serious liver infection transmitted through contact with infectious body fluids.
- HCV: Hepatitis C Virus, which can cause chronic liver disease, also transmitted via blood.

Antibiotics: Medications that destroy or inhibit the growth of bacteria, effective against bacterial infections.
- Antibiotic Resistance: A critical issue where bacteria evolve to resist the effects of antibiotics, making infections harder to treat.
- Nucleic Acid Analogues: Drugs that mimic the nucleic acids of pathogens, used in treatments against viral infections.

Emerging Infectious Diseases: New diseases that have recently appeared within a population.
Reemerging Infectious Diseases: Diseases that have returned after a significant decline in incidence.

Measles: A highly contagious viral disease characterized by a rash and flu-like symptoms.
Mumps: A viral infection primarily affecting salivary glands, preventable by vaccination.
Rubella (German measles): A contagious viral infection preventable by vaccination, known for causing rashes and mild fever.
Whooping Cough: A highly contagious respiratory disease caused by bacteria, preventable through vaccination.
Diphtheria: A serious bacterial infection affecting the mucous membranes of the throat, preventable by vaccination.
Tetanus: A bacterial infection characterized by muscle stiffness and spasms, preventable through vaccination.
Chickenpox: A contagious viral infection causing an itchy rash, preventable with a vaccine.
Haemophilus influenzae type b: A bacteria that can cause severe infections, particularly in children, preventable through vaccination.
Poliomyelitis (Polio): A viral disease that can cause paralysis, preventable through vaccination.
Streptococcus pneumoniae: Bacteria that can cause pneumonia and other infections, preventable through vaccination.

Pathogenic Microorganisms: Illustration includes the following:
- (A) Bacteria
- (B) Viruses
- (C) Protozoa
- (D) Helminths
Images of Diseases:
- Figure 3-6: Measles rash (© Lowell Georgia/Science Source)
- Figure 3-7: Chickenpox rash (Centers for Disease Control and Prevention [CDC]/Ann Cain)