Communicable Diseases and Infection

Communicable Disease

caused by an infectious agent or its toxic products and can pass from one person to another either

• directly (contact with an infected person) or

• indirectly (contaminated objects, vectors, food, or water).

Examples are influenza, tuberculosis, and measles.

influenza, tuberculosis, and measles.

Example of Communicable Disease

Infectious Disease

any disease caused by the invasion and multiplication of microorganisms (bacteria, viruses, fungi, or parasites) that damage or alter the host’s normal functioning.

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All communicable diseases are infectious, but not all infectious diseases are easily communicable. For example, tetanus is

infectious but not communicable because it does not spread from person to person.

Infection

The entry and multiplication of an infectious agent in or on the tissue of a host.

Colonization

The agent is present but does not injure the host’s cells or tissues.

Symptomatic Infection

The pathogens multiply and cause clinical signs and symptoms.

Pathogen

An organism that can cause disease.

Nonpathogenic Organism

An organism that does not cause disease.

Virulence

A measure of how likely an organism is to cause disease.

Contact

Any person or animal in close association with an infected person, animal, or freshly soiled materials.

Habitat

The place where an organism is usually found.

Reservoir

One or more species of plant or animal in which an infectious agent lives and multiplies to survive, and from which it can be transmitted to humans.

Carrier

A person who harbors the organism and can transmit it to a susceptible host, often without showing symptoms (asymptomatic).

• Types: incubatory, convalescent, intermittent, and chronic.

Incubatory

Convalescent

Intermittent

Chronic

Types of Carriers

Susceptibility and Pathogenecity

Reasons for Infection

• pathogenicity

• susceptibility of the host

• environment

Whether an infection becomes a disease depends on the

• _______ of the microorganism (its ability to cause disease)

• __________ (how vulnerable a person is), and the

• ________ The following factors explain why infections persist:

Antibiotic Resistance

Reasons for Infections: Susceptibility and Pathogenicity

Some bacteria develop resistance to commonly used antibiotics, which makes treatment difficult and prolongs infection.

Viral Variability

Reasons for Infections: Susceptibility and Pathogenicity

Some viruses, like influenza, have many strains and mutate often, so one vaccine cannot give full protection.

Resistance to Antiviral drugs

Reasons for Infections: Susceptibility and Pathogenicity

Many viruses are naturally resistant to antiviral medicines, which limits treatment options.

Emergence of New Agents

Reasons for Infections: Susceptibility and Pathogenicity

New infectious diseases, such as HIV and coronaviruses, appear from time to time and threaten global health.

Microbial Localization

Reasons for Infections: Susceptibility and Pathogenicity

Some microbes live in hard-to-reach areas of the body, such as the bones or central nervous system, and are harder to treat.

Opportunistic Infections

Reasons for Infections: Susceptibility and Pathogenicity

Organisms that are normally harmless can cause disease in

immunocompromised people (e.g., those with HIV/AIDS or undergoing chemotherapy).

Lack of Vaccination

Reasons for Infections: Susceptibility and Pathogenicity

People who are not vaccinated remain vulnerable to preventable diseases.

Global Travel

Reasons for Infections: Susceptibility and Pathogenicity

Air travel lets pathogens spread across long distances within hours, causing outbreaks in crowded areas.

Bioterrorism and Biologic Warfare

Reasons for Infections: Susceptibility and Pathogenicity

The intentional release of pathogens, such as anthrax or plague, threatens public safety.

Medical Intervention

Reasons for Infections: Susceptibility and Pathogenicity

Immunosuppressive drugs and invasive procedures increase the risk of infection, as does belonging to a high-risk group (the very young and the elderly).

Incubation period

Prodromal period

Period of Clinical Illness

Convalescence period

An infectious disease develops in four stages:

Incubation Period

The time between exposure to the pathogen and the first symptoms. The organism multiplies silently.

Prodromal period

A short phase of vague, nonspecific symptoms that signals the start of illness.

Period of Clinical Illness

Active disease with rapid pathogen multiplication, peak symptoms, and the highest risk of transmission.

Convalescence period

The recovery phase, when the host regains strength and returns to baseline health.

epidemiological triad of agent, host, and environment

Disease occurrence is best explained by the _____________

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Agent

The infectious microorganism, described by its pathogenicity,

infectivity, virulence, and infective dose.

Host

Susceptibility depends on age, sex, genetic makeup, immune status, and comorbidities. A host may be a patient, a carrier, a suspect, or a contact.

Environment

External factors that affect disease occurrence:

• physical (climate, geography),

• biological (plants, animals), and

• socio-economic (poverty, sanitation, access to health care).

Chain of infection

Infection spreads through a series of linked steps called the____

Infectious agent

Reservoir

Portal of exit

Mode of Transmission

Portal of entry

Susceptible host

Chain of Infection

Infectious agent

Chain of Infection

Microbes that can cause disease (e.g., bacteria, viruses, fungi,

parasites, amoeba).

Reservoir

Chain of Infection

Where the agent lives and multiplies: humans (including

carriers), animals, or the environment.

Portal of exit

Chain of Infection

How the agent leaves the reservoir: through the

• respiratory,

• gastrointestinal, or

• genitourinary tract, the

• skin, or the

• placenta.

Mode of transmission

Chain of Infection

The link that is easiest to break. It includes

• direct or indirect contact,

• droplet, airborne,

• vehicle-borne, vector-borne, and vertical transmission (mother to child).

Portal of entry

Chain of Infection

The same routes as the portals of exit, now allowing the agent to

invade a new host.

Susceptible host

Chain of Infection

A person who can be infected, determined by

• immunity,

• nutrition,

• health status, and

• environmental exposures.

Example: malnourished children are susceptible to infections.

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Infections differ in source, manifestation, and how they are acquired. Knowing these differences supports diagnosis, prevention, and control.

Subclinical / Asymptomatic

Latent

Exogenous

Endogenous

Healthcare associated infections (HAIs) / Iatrogenic

Types of Infection

Subclinical / Asymptomatic

The person is infected (confirmed by laboratory tests) but has no signs or symptoms. Such individuals may still spread the disease unknowingly.

• asymptomatic carriers of COVID-19, Hepatitis B, or HIV.

Latent

The microorganism stays dormant in the host, sometimes for years, without causing illness. It may reactivate and cause disease when triggered (e.g., by weakened immunity).

• herpes simplex virus, tuberculosis.

Exogenous

Caused by microorganisms that enter the body from outside sources such as contaminated food, water, air, or contact with infected people.

• Example: Salmonella infection from contaminated food.

Endogenous

Occurs when normally harmless microorganisms (normal flora) inside the body become pathogenic because of an imbalance or weakened immunity.

• Example: urinary tract infection (UTI) caused by E. coli from the gut.

Healthcare Associated infections (HAIs) / Iatrogenic

Formerly called nosocomial infections. These are acquired by patients while receiving care in hospitals or other healthcare facilities, often from invasive procedures, prolonged hospitalization, or cross-infection between patients and staff.

• Examples: MRSA (methicillin-resistant Staphylococcus aureus)

• ventilator- associated pneumonia.

Endemic

Epidemic

Pandemic

Sporadic

Classification of Disease by Distribution

Endemic

Constantly present in a particular geographic area or population group.

• Example: malaria in tropical countries; schistosomiasis in Samar and Leyte (Philippines).

Epidemic

Cases rise suddenly above the expected number in a specific area or community.

• Example: a cholera outbreak in an evacuation site after a typhoon.

Pandemic

An epidemic that spreads across several countries or continents and affects many people.

• Example: COVID-19; HIV/AIDS.

Sporadic

Occurs irregularly and infrequently, with only a few cases and no predictable pattern.

• Example: tetanus, gas gangrene.

Emerging infectous diseases

are illnesses that have recently appeared in a population or are rapidly increasing in number or geographic range, often because of newly identified pathogens or known agents spreading to new areas (e.g., HIV, SARS, COVID-19).

Re-emerging infectious diseases

________ had declined because of control measures such as vaccination and treatment but are now returning because of

• drug resistance,

• vaccine hesitancy, or

• environmental change

• e.g., tuberculosis, dengue, measles, polio).

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In short, emerging diseases are new threats, while re-emerging diseases are old threats returning.

1. crowded urban living and demographic shifts

2. international travel and migration,

3. socio-economic problems,

4. environmental and climate change,

5. misuse of antibiotics, and vaccine hesitancy.

Emerging & Re-emerging Infectious disease

Both are driven by

• ___(5)