Infectious Diseases – Basic Knowledge and Considerations

1. Identifying an Epidemic

• To determine if an epidemic is occurring, consider:

• The usual frequency of the disease in the community: Compare the current number of cases with the average number of cases in the same period in previous years.

• Previous appearance of the disease: Identify if the time of the disease’s appearance is unusual compared to its typical cyclical pattern.

• The comparison period depends on:

• The disease cycle: Each disease has a different cycle. For instance, seasonal influenza typically appears in winter.

• Artificial intervention: Preventive and treatment measures can impact disease cycles.

• Disease development process in the community: Determine the speed at which the disease is spreading within the community.

• Epidemic Year Factor:

• This index hints at whether a particular year is an epidemic year.

• Epidemic Multiplier Factor (EMF):

• Formula: EMF = (Number of new cases in 12 months) / (Number of new cases in multiple years / Number of months)

• Significance: Compares the number of new cases in the current year with the average number of new cases over multiple years.

• Epidemic Growth Factor (EGF):

• Formula: EGF = (Number of new cases in the month / Number of days in the month) / (Number of cases in a year / 365 days)

• Significance: Compares the number of new cases in the current month with the average number of new cases in a year.

• Primary Attack Rate:

• Formula: Primary attack rate = (Number of new cases in a period) / (Number of susceptible individuals in the same period)

• Significance: Determines the infectivity of the disease in the community.

• Secondary Attack Rate:

• Formula: Secondary attack rate = (Number of new cases added) / (Number of susceptible individuals remaining = Total number of susceptible individuals – Number of cases already occurred)

• Significance: Determines the ability of the disease to spread from infected individuals to healthy ones.

2. Infectivity and Virulence

• Infectivity: The ability of the pathogen to invade and multiply within the host.

• Infectivity is measured by: Secondary attack rate.

• Pathogenicity: The ability of the pathogen to produce disease symptoms.

• Pathogenicity is measured by: The proportion of infected people who show symptoms.

• Virulence: The severity of the disease caused by the pathogen.

• Virulence is measured by: Case fatality rate (CFR).

3. Chain of Infection

• Chain of infection: Includes the pathogen leaving the source (“Portal of exit”), transmission through the environment (“Mode of transmission”), and entry into a new host (“Portal of entry”).

• Source of infection: Three main groups:

• Human: Sick individuals or carriers.

• Animals: Animals can be hosts to pathogens.

• Environment: The environment can harbor pathogens, such as soil, water, and air.

• The most significant source of infection: Carriers who are not ill.

• Individuals can still transmit diseases after recovery in cases of: Diphtheria, hepatitis, and typhoid fever.

• According to the WHO, approximately 70% of infectious diseases are transmitted from animals to humans.

• Zoonosis: Diseases transmitted from wild animals to humans.

• Environmental sources of infection:

• Histoplasmosis fungus: Transmitted through the respiratory system from infected soil.

• Legionella pneumonia bacteria: Transmitted through the respiratory system from contaminated water sources.

• Direct transmission: Through the air at short distances, saliva when coughing/sneezing.

• Indirect transmission: Through the air at long distances, dried saliva droplets.

4. Host

• Host: The third link in the chain of infection.

• Host factors to consider:

• Site of infection: Where the pathogen enters and develops in the body.

• Incubation period: The time from infection to the appearance of symptoms.

• Resistance: The ability of the body to fight off the pathogen.

• Example of a pathogen with a secondary site of infection: Poliovirus enters through the gastrointestinal tract and then damages the grey matter of the anterior horn of the spinal cord.

5. Community Immunity

• Community immunity: The widespread presence of immunity within a community.

• Diphtheria: Requires a community immunity rate of 75%.

• Rubella: Requires a community immunity rate of 85-90%.

6. Mechanisms of Infection

• The chain of infection only works if its stages are continuous, thanks to the mechanisms of infection.

• Mechanisms of infection include three stages:

• Pathogen excretion from the old host: The pathogen escapes the old host through the “portal of exit.”

• Pathogen survival in the external environment: The pathogen exists in the environment and awaits an opportunity to enter a new host.

• Pathogen entry into a new host: The pathogen enters the new host through the “portal of entry.”

7. Site of Infection

• Site of infection: The organ or tissue in the body where the pathogen can exist and develop.

8. Infection

• Infection is an ecological phenomenon, representing the interplay between two living entities belonging to different classes.

• Acute infectious disease is an imbalance of the immune-infective ecological equilibrium in the process.

Note:

• Knowledge about infectious diseases is crucial to prevent and control them effectively.

• Stay updated on disease information from reputable sources.

• Practice appropriate prevention measures for each disease.

• If you experience symptoms, seek medical attention for timely diagnosis and treatment.