Infectious Disease Epidemiology😊

Infectious Disease Epidemiology

Timings in Spread of Infection

Time Variables

• Incubation Period: Time between
• entry of organism
• and first signs and symptoms.

Disease	Incubation period (Days)
Chicken pox	10–21
Measles	8–16
Rubella	14–21
Hepatitis A	15–50
Covid-19	3–12

• Incubation period:
• Entry → first symptom in an individual

• Median incubation period:
• Entry → disease in 50% cases

• Generation Time: Time between
• entry of organism
• and maximum communicability.

• Serial Interval: Time between
• 1° case
• and 2° case.

• Latent Period: Time between
• onset of risk factor
• and signs and symptoms of a non-communicable (chronic) disease.

• Isolation:
• Done for clinical cases.
• Duration: Till maximum communicable period.
• Not incubation period

Disease	Duration of isolation
Chickenpox	until all lesions are crusted (About 6 days after onset of rash)
Measles	7 days after onset of rash
Hepatitis A	3 weeks of onset
Tuberculosis	until 3 weeks of effective chemotherapy
COVID	14 days
Meningococcal Meningitis	until the first 6 hours of effective therapy are completed
Streptococcal Pharyngitis	---
Diphtheria	>2 culture reports 24 hours apart

• Quarantine:
• Done for apparently healthy individuals.
• Duration: maximum incubation period.

Source Variables

• Reservoir:
• Natural habitat where infectious agent lives and multiplies.

• Source:
• Natural habitat where infectious agent lives, multiplies and transmits.

• Host:
• Primary/Definitive Host:
• Sexual maturation occurs.
• Secondary/Intermediate Host:
• Asexual development occurs.

ㅤ	Primary host	Secondary host
Filaria	Man	Culex mosquito
Malaria	Anopheles mosquito	Man
Tapeworm	Man	Pig (T. Solium), Cattle (T. Saginata)
Guinea Worm	Man	Cyclops
Hydatid Disease	Dog	Sheep, Cattle, Man
Sleeping Sickness	Man	Tsetse Fly

• Mnemonic: Does not Hide in Man

• Type of Host:

ㅤ	Case	Subclinical case	Carrier
Risk factor	⊕	⊕	⊕
Pathology	⊕	⊕	⊕/⊖
Signs/Symptoms	⊕	⊖	⊖
Transmission	⊕/⊖	⊖	⊕

Carrier Stages

• Stages of a Disease Process:
• Entry
• Incubation period
• Period of communicability
• Clinical stage
• Recovery/Convalescent phase

Types of Carriers

• Healthy Carrier:
• Arise from a subclinical case or a non-virulent strain.

• Other types of carriers:
• Convalescent carrier
• Incubatory carrier.

Types of Cases

• Primary case:
• First case in community.

• Secondary case:
• Subsequent cases which arise from the primary case.

• Index case:
• First observed case in the community, by clinician

Attack Rates

• Attack Rate:
• AKA Special incidence rate.
• Total number of cases x 100
  Total susceptible population

• Secondary Attack Rate (SAR):
• Does not account for time.
• Total number of 2˚ cases in 1 IP x 100 (%)
  Total susceptible population - 1˚ cases

Modes of Transmission

• Chain of Infection:
• Infectious agent
• Reservoir → source
• Portal of exit
• Mode of transmission
• Portal of entry
• Susceptible host

• Inhibition of disease transmission:
1. Interrupt the transmission:
• Weakest line of chain of infection.
2. Protection of host:
• Vaccines.

Types of Transmission

Airborne mode of Transmission

• Droplets:
• Range: 30-60 cm.
• Direct mode of transmission.

• Droplet Nuclei:
• Dried residue of fine droplets.
• Indirect mode of transmission.
• Airborne
• Airborne precautions:
• TB
• Measles
• SARS/Covid
• Chicken pox
• Influenza

Biological Transmission

Levels of Prevention

Epidemic Variables

Exotic disease vs Epidemic

• Exotic disease
• Disease not found in india → brought from another country
• 25-year-old male who was diagnosed with the Yellow fever → upon arrival at an Indian airport → leading to quarantine

• Epidemic
• Occurrence of a single case of a previously eliminated disease
• A case of polio in India
• Occurrence of a single case of an exotic disease
• Yellow fever in India

Types of Epidemics

Single Source/Common Source:

• Single Exposure Point Source



• Rapid rise & rapid fall
• Explosive in nature.
• All cases within 1 incubation period.
• Eg: Epidemic food poisoning

• Single source multiple exposure
• Outbreak of cholera

• Multiple Exposure Point Source
• AKA continuous exposure epidemic.
• Rapid rise with persistent high no. of cases.
• Endemic presence.

• Multiple Sources (Propagated Epidemic):
• Cases, large outbreaks.
• Person → Person transmission.
• Multiple waves.

Disease Phases & Basic Reproduction Number

Control, Elimination & Eradication

• Diseases Eradicated:
• Worldwide: Smallpox
• India:
• Smallpox
• Dracunculiasis (Guinea worm disease)
• Yaws
• Wild poliomyelitis
• Mnemonic: Small Wild Dracula Yawns

Disease Elimination in India vs Targeted for Elimination

Basic Reproduction Number

• Average number of new cases arising from a single case.

• Practical utility for spread of infection.

• Based on:
• Duration of infectiveness.
• Transmission per contact b/w a susceptible & infected individual.
• Average rate of contact b/w susceptible & infected.

• Inference:
• R0
• = 1: Infection levels are maintained.
• > 1: Epidemic ↑
• < 1: Number of cases will ↓

Basics of Epidemiology

Epidemiological Models

Epidemiological Triad:

• Agent

• Host

• Environment

Epidemiological Triangle:

• Agent

• Host

• Environment

• Time

Advanced model of triangle of epidemiology
(Most modern concept of epidemiology):

Iceberg Phenomenon of Disease

• Apparent cases: Diagnosed & clinical.

• Inapparent cases:
• Carriers
• Subclinical
• Non-virulent cases
• Undiagnosed cases

• Diseases that do not show iceberg phenomenon
• Mnemonic: MTR
• Measles
• Tetanus
• Rabies
• Mnemonic: Do not put ice in Meals (Measles), Tea (Tetanus) and Raggi (Rabies)

• Diseases that show iceberg phenomenon
• Mummy Poli (Polio) Influenzar () anu → Dye (diphtheria) cheyth Ruby (Rubella) itt Japanil (JE) poi → AB (Hep AB) Type (Typhoid)
• Polio
• Mumps
• Japanese encephalitis
• Influenza
• Diphtheria
• Typhoid
• Hepatitis A & B
• Rubella

Important Terms

RE α 1 / (Precision α Reproducibility/ Reliability)

• Precisely () random () alkkare kuthiyal reproduce () cheyyam

SE α 1 / (Accuracy α Validity)

• Vaccurat (Validity → Accuracy) systematic (systematic error)

• Validity:
• Hit the board
• Results are within a desired range.

• Accuracy:
• Bull’s eye
• Nearness/closeness to the actual true value.

• Reliability:
• Repeatability
• Out of the board, but same result when repeated
• Dependable, reproducible, repeatable results.

• Note:
• Serial interval:
• Proxy indicator for incubation period.
• CFR:
• Denote virulence of a disease.