Infectious Disease Concepts Chapter 2

Chapter 2: Basic Concepts of Infectious Disease

• The chapter cover infectious diseases, focusing on basic concepts like pathogenicity, virulence, and transmission.
• The objectives are: describe relationships among a host, microbiome, and pathogens; apply concepts of pathogenesis to specific infections; discuss how infectious diseases impact communities and emerging pathogens.
• Case study of Brandon, a stockbroker with symptoms of a sexually transmitted disease, syphilis.
• Syphilis symptoms: small lesion, fluid leakage at the tip of the penis.
• Diagnostic test: clinical lab found corkscrew-shaped bacteria (Treponema pallidum).
• Treatment: long-acting penicillin, quick lesion resolution.
• Contact tracing of Brandon's sexual partners.

2.1 Normal Microbiota versus Pathogens

• Objectives: describe differences between normal microbiota and pathogens; relationship between infection and disease, and virulence vs. pathogenicity; infectious dose vs. lethal dose; and attributes of a successful pathogen.
• Microbes colonize many areas of the human body. This collection is known as human microbiota.
• Microbes colonizing body surfaces remain attached and replicate.
• Adhesins allow microbes to attach to host cells.
• Parasites are microbes that cause harm inducing infections. Two types are ectoparasites and endoparasites.
• Pathogen: a bacterium, virus, fungus, protozoan, or worm that causes disease in humans.
• Pathogenicity: a pathogen's ability to cause disease.
• Infection occurs when a pathogen/parasite enters and begins to grow in a host. Not all infections cause disease.
• Disease occurs when the infection results in symptoms.
• Acute infection: rapid symptom development and resolution.
• Chronic infection: gradual symptom development and resolution. Tuberculosis is an example.
• Primary pathogens: likely to cause disease after infection.
• Opportunistic pathogens: less likely to cause disease in a healthy host.
• Low virulence is a trait of some opportunistic infections.

2.1 Pathogenesis

• Virulence describes the severity of harm caused by pathogens.
• Methods to measure virulence include: Lethal dose (LD50) and Infectious dose (ID50).

2.1 Pathogenesis

• Invasion is when a pathogen enters a living cell and lives there.
• Invasiveness is the ability of a bacterial pathogen to rapidly spread through tissues.

2.1 Host Range

• Host range refers to the specific animals a pathogen can infect and cause disease in.
• Some organisms can infect only one type of host, producing disease in that one host. Salmonella enterica serovar Typhi infects only humans.
• A broad range of organisms infects many species. Salmonella enterica serovar Typhimurium infects animals but also humans.

2.2 Basic Concepts of Disease

• Objectives: distinguish between signs and symptoms of a disease; explain the role of immunopathology in infectious disease; describe the five basic stages of infectious disease.
• Disease: disruption of normal structure or function of a body part, organ, or system that has recognized symptoms and signs.
• Infectious disease: a disease caused by a pathogen that can spread from one host to another.
• Signs of disease are objective markers; symptoms are subjective markers

2.2 Signs, Symptoms, and Sequelae

• Diseases are identified by characteristic signs and symptoms. Signs are examined objectively; symptoms are expressed subjectively.
• Many of the signs and symptoms of a disease are caused by the host's response to infection (immunopathology).
• Cells can cause damage to nearby host tissue when killing the microbe.
• Sequelae are pathological consequences after a disease resolves.

2.2 Stages of Infectious Disease

• Characteristic stages of acute infectious disease include: incubation phase; prodromal phase, illness phase; decline phase; convalescence phase; and long-term phase.

2.2 Disease Transmission

• Public health agencies track disease rates and severity.
• Morbidity: rate of illness due to a disease.
• Mortality: rate of death due to a disease.
• The CDC publishes the Morbidity and Mortality Weekly Report.

2.3 Infection Cycles and Disease Transmission

• Objectives: describe complex vs. simple infection cycles; differentiate between endemic, epidemic, and pandemic disease, and animal reservoirs and incubators.
• Infection cycle: route of transmission between hosts.
• Horizontal transmission: transfer between hosts.
• Vertical transmission: transfer from parent to offspring.
• Endemic disease occurs at a low rate continuously in a community, often in an animal reservoir.
• Epidemic disease: an increase in the number of cases in a community in a short period.
• Pandemic disease: a worldwide epidemic.

2.3 Case Study: Emma Katherine and Rocky Mountain Spotted Fever

• Emma Katherine developed sickness following a hiking and camping trip to North Carolina.
• Emma noticed a rash, and the PA suspected RMSF.
• The tick bite may have been the cause.

2.4 Zoonotic Diseases

• Pathogens impacting animals, potentially spreading to humans.
• Transmission: Direct or indirect "spillover."
• Lyme disease: pathogen spreads to humans after the tick bite.
• Ebola: virus present in animals and humans.
• Animal and insect reservoirs are "incubators" for new diseases. Two different strains of the influenza virus can infect an animal at the same time. The pathogens swap segments of their genomes and develop into a new more dangerous virus.

2.4 Disease Portals and Biosafety

• Portals of entry and exit (e.g. respiratory, fecal-oral, skin, urogenital, or parenteral).
• Biosafety precautions (e.g., proper hand hygiene and PPEs).
• Different lab settings use various levels of agent containment (BSL 1-4) to protect medical personnel, and others.

2.5 Host Factors in Disease

• Host Factors: biological features of humans impacting infections' course.
• Lifestyle impacting susceptibility to disease (e.g. age, hygiene, diet, and exercise).
• Existing conditions (e.g., chronic infections, other diseases).
• Immune status impacting susceptibility to and the outcome of infection.
• Occupation and exposure increase risk of infection (e.g., healthcare workers, agricultural workers).

2.6 Global Change and Emerging Infectious Diseases

• Objective: Explain how civilization has impacted emerging infectious diseases and how climate change alters infectious patterns.
• Emerging diseases: new diseases recently appearing in populations
• Reemerging diseases: known diseases increasing in incidence.
• Human activities bring humans closer to disease reservoirs and vectors, impacting disease increases.

2. Case Study: The Third Pandemic

• A historical case study of the Third Pandemic of Bubonic Plague during 1884.
• Symptoms of the Bubonic Plague: a swollen, dark lesion or "bubo" appeared at first.
• Spread: through the blood stream in individuals.
• Transmission: rats housed the bacterium via flea vectors.
• Several pandemics of plague have occurred over the centuries, including the Black Death.

2. Are All Infectious Diseases Known to Us?

Emerging/re-emerging diseases are continually being discovered. Global increases in disease are caused by human activities and interaction with disease vectors and reservoirs. Deforestation may play a role in the spread of infectious diseases.

Explore the fundamental principles of infectious diseases in this quiz based on Chapter 2. Understand concepts such as pathogenicity, virulence, and transmission through case studies, including syphilis. Analyze the relationships between the host, microbiome, and pathogens, and how these diseases affect communities.