Viruses

🦠 Viruses: Overview

• Noncellular: Not made of cells; can't live or reproduce on their own.

• Size: Smaller than 200 nanometers.

• Main Parts:

  • Capsid: Protein shell; may be:

    • Naked (no outer membrane)

    • Enveloped (has a membrane from the host cell)

    • Usually symmetrical

  • Nucleic Acid Core: DNA or RNA (single or double-stranded; linear or circular)

🧬 Virus Shapes (Morphology)

• Icosahedral: Round with 20 triangle faces (e.g., adenovirus)

• Helical: Rod-shaped, spiral protein coat (e.g., tobacco mosaic virus)

• Complex: Irregular with extra parts like tails (e.g., bacteriophages)

• Spherical: Round-looking due to envelope (e.g., influenza virus)

🧫 Virus Classification

• Based on:
  
  

  • Type of genetic material (DNA or RNA)

  • Single or double-stranded

  • Shape and size

  • Presence of envelope

• Baltimore Classification:
  
  

  • Group I: Double-stranded DNA

  • Group II: Single-stranded DNA

  • Group III: Double-stranded RNA

  • Group IV: Positive-sense single-stranded RNA

  • Group V: Negative-sense single-stranded RNA

  • Group VI: Single-stranded RNA with reverse transcriptase

  • Group VII: Double-stranded DNA with reverse transcriptase

🧪 Virus Characteristics

• Only reproduce inside living cells

• No metabolism or response to stimuli

• Evolve through mutations and natural selection

🧬 Bacteriophages (Viruses that infect bacteria)

• Attach to specific receptors on bacteria

• Inject genetic material into host

• Use host machinery to replicate

• Often evade immune detection

🧍‍♂ Viral Infections & Control

• Entry Points: Skin, respiratory tract, digestive tract, sexual contact

• Diseases:

  • Sexually transmitted: HIV/AIDS, genital herpes, warts

  • Childhood: Measles, mumps, chickenpox

  • Respiratory: Cold, flu

  • Skin: Warts, shingles

  • Digestive: Diarrhea, gastroenteritis

  • Nervous system: Rabies, polio, encephalitis

  • Other: Hepatitis, cancer, mononucleosis

• Treatment:

  • Antibiotics don’t work

  • Prevention: vaccines, antiviral drugs

🧬 Other Infectious Agents

• Viroids: Small, naked RNA strands (plant diseases)

• Prions: Misfolded proteins causing brain diseases (e.g., Mad Cow, Kuru)

🌍 Emerging & Re-emerging Diseases

• Emerging: New or rapidly spreading

  • Examples: AIDS, SARS, Ebola, Zika, Hantavirus, Chikungunya

• Re-emerging: Once controlled, now rising again

  • Examples: Cholera, malaria, tuberculosis, dengue, plague

🧫 HIV & AIDS

• Cause: Human Immunodeficiency Virus (HIV)

• Transmission:

  • Sexual contact

  • Blood transfusion

  • Shared needles

  • Mother to child

• Stats (2023):

  • 39 million people globally with HIV

  • 1.2 million in the U.S.

  • 65% of new cases in Sub-Saharan Africa

🤧 Influenza (Flu)

• Symptoms: Fever, cough, muscle aches, runny nose

• Types: A, B (seasonal), C, D

• Surface Proteins: Hemagglutinin (H) & Neuraminidase (N)

• Recent Trends:

  • 2017–18: Severe H3N2 season

  • 2018–19: H1N1 dominant

  • 2020–21: Low flu due to COVID-19

  • 2022–24: Moderate seasons with H1N1 & H3N2

• Risk: Higher chance of heart attack after infection

🦟 Zika Virus

• Spread by: Aedes mosquitoes

• Symptoms: Fever, rash, joint pain, red eyes (most people show no symptoms)

• Impact:

  • Brazil: ~3,500 microcephaly cases

  • U.S.: 21% rise in birth defects in affected areas

🦠 COVID-19 (SARS-CoV-2)

• Symptoms: Fever, cough, fatigue, loss of taste/smell, GI issues

• Spread: Respiratory droplets; 2–14 day incubation

• Infectiousness (R₀):

  • COVID-19: ~2–5.7

  • Measles: 12–18

  • HIV: ~4

• Origin: Likely from bats; first seen in Wuhan, China (2019)

Vaccination: Recommended for everyone 6 months and older (2024–25 season)