Topic 26- Infectious Diseases

I. Introduction to Infectious Disease

A. Why Do We Get Sick?

1. Virulence – ability of a pathogen to cause disease

2. Pathogen – organism that causes disease

3. Intrinsic factors – internal (host-related) causes

4. Extrinsic factors – pathogen-driven causes

5. Epidemic – localized outbreak of disease

6. Pandemic – worldwide outbreak

   • Examples:
     a) COVID-19
     b) HIV/AIDS
     c) Bubonic Plague
     d) Cholera

B. Types of Pathogens

1. Bacteria

2. Parasites

3. Viruses (and more)

II. Viruses and Influenza

A. Virus Basics

1. Structure:

   • Protein shell (capsid)

   • Genetic material (DNA or RNA)

2. Replication:

   • Require host cells

   • Copy genetic material using host machinery

   • Do not generate their own energy or perform metabolism

B. Influenza Virus

1. RNA virus

2. 8 genome segments → each codes for 1 gene

3. Surface proteins: Hemagglutinin (H) & Neuraminidase (N)

   • Example strains: H1N1, H3N2

4. Symptoms: fever, chills, aches, congestion

C. The Spanish Flu (H1N1)

1. Outbreak in 1918

2. Infected ~⅓ of world population

3. Over 20 million deaths

4. In Philadelphia: 12,000 deaths in 5 weeks

III. Lyme Disease and Malaria

A. Lyme Disease

1. Lifecycle:

   • Tick hatches → mouse

   • Tick molts → deer

   • Tick lays eggs

   • Humans = dead-end host

2. Treatment: antibiotics

3. Most common tick-borne disease in the U.S.

B. Malaria and Plasmodium

1. Parasites

• Live in/on host, feed off host nutrients

• Reproduce more rapidly than host

• Parasitic interaction = +/- (host is harmed)

2. Malaria Facts

• ~200 million new cases per year

• 409,000 deaths in 2019

• ⅔ deaths = children under 5

• Caused by Plasmodium

• Transmitted by Anopheles mosquitoes

3. Plasmodium Life Cycle

1. Mosquito bite → sporozoites enter human liver

2. Liver cells release merozoites → infect red blood cells

3. Some develop into gametocytes

4. Another mosquito bites → takes in gametocytes

5. In mosquito:

   • Gametocytes fuse → zygote (2n)

   • Meiosis → oocyst → new sporozoites

6. Cycle continues with next bite

4. Malaria Control

• Prevention: vector management (nets, barriers, behavior)

• Treatments and Vaccines:

  • RTS,S/AS01 (Mosquirix) – first approved in 2019

  • R21 (Oxford) – up to 77% efficacy

• World Malaria Day: April 25

IV. SARS-CoV-2 and COVID-19

A. What is SARS-CoV-2?

1. Enveloped RNA virus

2. One of 7 human coronaviruses:

   • 4 = common cold

   • Others: SARS (2003), MERS (2012)

3. Zoonotic origin – likely from bats

   • Other zoonotic examples: Ebola, HIV

B. The COVID-19 Pandemic

1. Spread: person-to-person via droplets/aerosols

2. Global spread in ~2 months

3. Reasons for rapid spread:

   • Possibly lower fatality = higher transmission

   • Case Fatality Rates (CFR):

     • SARS: ~10%

     • MERS: ~35%

     • COVID-19: ~0.5–5%

     • Flu: ~0.1%

   • Potential tradeoff between virulence and transmissibility

C. COVID-19 Control Goals & Strategies

1. Decrease R (reproduction number) < 1

2. Contact tracing and isolation

3. Develop effective treatments to lower fatality

What Works:

• Social distancing, quarantine, lockdown

  • Goal: reduce R (ex: R = 2 → R < 1)

• Testing + contact tracing + quarantine

  • Goal: R <<< 1

• Better treatment protocols

  • Early: treated like ARDS

  • Now: treat hypoxia, prevent blood clots

• Antiviral treatments

  • Goal: reduce CFR/IFR

  • Ongoing trials (some success)

  • Example: Tamiflu (for influenza)

• Vaccines

  • mRNA (e.g. Pfizer, Moderna)

  • Viral vector (e.g. Johnson & Johnson)

  • Very high effectiveness