Public health

Types of Infection

• Cases show manifestations of disease and are infectious for a varied period of time depending on the nature of the disease and whether specific therapy is available and properly given or not.
• Untreated cases are usually infectious throughout the clinical course of the disease, which varies in different diseases.

Carriers

• A carrier is an apparently healthy person who is infected and harbors a pathogenic organism in their body, without showing manifestations of disease, but is infectious and can spread infection.
• Importance of carriers: they are clinically healthy, not known, move freely, are a large number, and are a dangerous group.
• Types of carriers:
  • Incubatory carrier: infectious during the incubation period.
  • Convalescent carrier: a certain percentage of convalescents (recovered cases) continue to harbor the causative organism in particular foci for a varied period of time.
  • Contact carrier: contacts of cases or carriers of some infectious diseases may be infected but not diseased.
  • Healthy carrier: inhabitants of endemic areas of infectious diseases may get infected from a polluted environment.

Modes of Transmission

• Ingestion infection (food-borne infection)
• Arthropod-borne infection (insect-borne infection, vector-borne infection)
• Contact infection (including percutaneous infection)
• Occasional modes of transmission:
  • Injection infection: a form of contact infection
  • In-utero infection: congenital infection, vertical infection (including transplacental infection)

Entry of Infection

• Respiratory tract: for respiratory (droplet) infection
• Gastrointestinal tract: for ingestion infection
• Percutaneous route: for arthropod-borne infection, wound infection, and injection infection
• Skin and mucous membranes: for other contact infections
• Fetal circulation: for in-utero infection

Droplet Infection

• Modes of transmission:
  • Direct droplet infection: both the reservoir of infection and the new host are found together within the same place
  • Indirect droplet infection: reservoirs and hosts do not come together, infection is transmitted by airborne infection, contaminated articles, and fomites

In-Utero Infection

• It is infection transmitted from the pregnant to the fetus either:
  • Early infection in the first trimester, before formation of the placenta
  • Trans-placental infection, after formation of the placenta
• Hazards of in-utero infection: unfavorable outcome of pregnancy that may be either:
  • Lethal: abortion, stillbirth, or neonatal mortality
  • Sublethal: congenital anomalies and malformations (deformities)

Vertical Infection

• Mother to infant infection, called “vertical infection”
• Vertical infection is transmitted in three ways:
  • In-utero infection
  • Perinatal infection: infection acquired from the infected birth canal during childbirth
  • Breast-feeding: infection through nursing (lactation of) the baby (CMV-HIV-HBV& HCV)

Exposed Host

• The host is the third link of the infectious cycle
• When infected, infection either passes unnoticed or is followed by disease, according to certain agent and host factors
• Host factors are related to body resistance (nonspecific) and immunity (specific) factors

Defense Mechanism of the Body

• Defense mechanism is made of two components:
  • Natural barriers of infection:
    • Healthy epithelial surfaces (skin and mucous membranes) form the first line of defense that prevents invasion of the body by organisms
    • Skin: intact surface is a natural barrier (undamaged), bactericidal effect of sweat
    • Respiratory tract: defense is due to hair, cilia of epithelial lining, sneezing, and coughing help elimination of infection

Community Control Measures

• Applied preventive measures: measures of primary prevention related to particular disease involving a group or community are applied
• Control of environment: control of vehicles and vectors, e.g., adequate ventilation and spacing, super chlorination of water supply, and sanitary collection and disposal of community
• Health education: specific prevention
• Control measures:
  • Case-finding and control of cases and contacts
  • Epidemiologic study and investigation to trace sources and channels of infection
  • Drastic control measures: to be taken if necessary, e.g., closing schools, and food markets

Eradication of Infectious Disease

• It is getting rid of the causative organism, and consequently of the disease, in a certain area, country, or worldwide: no reported cases, nor reservoirs of infection
• Eradication, however, has been achieved for a limited number of infections, otherwise it is practically difficult to fulfill, especially in developing countries