Introduction to Infectious Diseases

Definition

• Infectious diseases are illnesses caused by infectious agents such as viruses, bacteria, parasites, or fungi.
• Historically a major cause of death worldwide, with significant epidemics including smallpox, TB, syphilis, cholera, and plague.

Historical Impact

• 1918 influenza pandemic resulted in approximately 40 million deaths globally.
• Advances in nutrition, antibiotics, immunization, food safety, housing, and sanitation have reduced the prevalence of infectious diseases.

Current Impact

• As of 2012, infectious diseases accounted for three of the top 10 causes of death globally:
  • Lower respiratory tract infections
  • HIV/AIDS
  • Diarrheal diseases
• Continual issues with diseases like malaria, tuberculosis, Ebola, and MERS-CoV.

Infection Process

• Incubation Period: The time from infection to onset of symptoms.
• Clinical Stage: Marked by the onset of symptoms, which can range from mild to severe.
• Resolution: The disease lasts until resolution of infection or death of the individual.

Important Terms

• Infectious Period: Duration in which an infected person can spread the disease.
• Carriers: Individuals who can spread the disease without showing symptoms.
• Case Fatality Rate: Proportion of deaths from a disease among the total infected.
• Basic Reproductive Rate (R0): Average number of secondary cases per infected individual (e.g., measles R0 can be up to 15).
• Secondary Attack Rate: Proportion of exposed individuals who develop the infection (e.g., about 10% for seasonal influenza).

Categories of Infectious Diseases

• Zoonotic Diseases: Transmitted from animals to humans, making up over 60% of human infectious diseases. Examples include:

  • Leptospirosis
  • Toxoplasmosis
  • Rabies

• Emerging Infectious Diseases: New diseases or diseases increasing in incidence or geographical range. Examples include:

  • MERS-CoV
  • Ebola
  • New influenza strains

• Neglected Tropical Diseases: Affect the poorest regions and lack global attention. Examples include:

  • Leishmaniasis
  • Trachoma
  • Sleeping sickness

• Vector-Borne Diseases: Transmitted via vectors like mosquitoes, ticks, and fleas. Account for 17% of all infectious diseases. Examples include:

  • Malaria
  • Dengue
  • Schistosomiasis

Conclusion

• Infectious diseases remain a significant global health issue, with various categories and terms essential for understanding their impact and management.