Lecture Notes: Infectious Diseases

Introduction to Infectious Diseases

• Definition: Diseases caused by infectious agents like viruses, bacteria, parasites, or fungi.
• Historical Impact: Previously the leading cause of death globally.
  • Large epidemics: Smallpox, TB, syphilis, cholera, plague.
  • 1918 Influenza pandemic: ~40 million deaths.
• Improvements: Advances in nutrition, antibiotics, immunization, food safety, housing, sanitation have reduced infectious diseases.
• Current Impact: In 2012, responsible for 3 of the top 10 global causes of death.
  • Lower respiratory tract infections, HIV/AIDS, diarrheal diseases.
  • Persistent issues: Malaria, tuberculosis.
  • Emerging diseases: Ebola, MERS-CoV.

Infection Process and Terms

• Infection Response: Varies based on immune system strength and immunity.
  • Weakened immune systems (due to drugs, age, or other diseases) are more susceptible.
  • Lack of prior exposure or vaccination increases susceptibility.
• Incubation Period: Time from infection to symptom onset.
• Clinical Stage: Period marked by symptom onset, varying in severity.
• Infectious Period: Time during which the infected person can spread the disease.
• Carriers: Infected individuals who can spread disease without showing symptoms.
• Case Fatality Rate: Proportion of infected individuals who die.
• Basic Reproductive Rate (R₀): Average number of secondary cases from one infected individual (e.g., measles R₀ = 15).
• Secondary Attack Rate: Proportion of exposed individuals who develop the infection (e.g., influenza ~10%).

Categories of Infectious Diseases

• Zoonotic Diseases: Transmitted from animals to humans.
  • Examples: Leptospirosis, toxoplasmosis, rabies.
• Emerging Infectious Diseases: Newly appearing or rapidly increasing diseases.
  • Examples: MERS, Ebola, new influenza strains.
• Neglected Tropical Diseases: Affect the poorest countries, historically under-recognized.
  • Examples: Leishmaniasis, trachoma, sleeping sickness, schistosomiasis.
• Vector-Borne Diseases: Transmitted via vectors like mosquitoes, ticks, flies, fleas, snails.
  • Examples: Malaria, dengue, sleeping sickness, schistosomiasis.
  • Impact: Over 17% of infectious diseases, >1 million deaths annually.

Conclusion

• Overview of infectious diseases, relevant terms, and different types of diseases.