305 week 7 epidemiology and disease

Infection

The entry, establishment, multiplication, and invasion of a host’s normal anatomical barriers by a pathogen, resulting in disease.

Periods of disease

Incubation, prodromal, period of illness, period of decline, period of convalescence

Factors affecting infection

Virulence, transmissibility, host susceptibility, and viral load.

Colonization

A pathogen enters and multiplies without invading tissues or causing disease.

Normal flora (commensals)

Microorganisms normally limited to particular anatomical sites, such as the intestine.

Do colonizers have pathogenic potential?

Yes

Epidemiology

The study of the distribution and determinants of health-related states and events in populations and the application of this knowledge to control health problems.

Host factors

Age, gender, occupation, pregnancy, personal habits

Chain of infection

Modes of transmission → portal of entry → susceptible host → pathogen → reservoir → portal of exit.

Modes of transmission

The pathways through which pathogens spread between hosts, either directly or indirectly.

Direct transmission

Spread through direct contact, respiratory droplets, soil contact, inoculation, or transplacental transmission.

Indirect transmission

Spread through vehicles, vectors, airborne particles, fomites, or contaminated hands and fingers.

Direct contact transmission

Spread through skin or mucosal contact, including kissing, sexual contact, and unclean hands.

Examples of direct contact transmission

Common cold, skin infections, eye infections, and sexually transmitted infections.

Fomites

Inanimate objects that carry pathogens, such as doorknobs and towels.

Examples of fomite-associated pathogens

E. coli, Salmonella, adenovirus, and coronavirus.

Respiratory droplet transmission

Spread through respiratory droplets larger than 5 μm produced by coughing, sneezing, or talking; happens with close contact under 3 feet.

Examples of droplet-transmitted diseases

Diphtheria, Haemophilus influenzae infection, meningococcal infection, pertussis, COVID-19, influenza, and rubella.

Aerosol transmission

Spread through particles smaller than 5 μm that can remain suspended in the air for long periods. Produced by coughing, sneezing or talking

Examples of aerosol-transmitted diseases

Tuberculosis, measles, varicella (chickenpox), and smallpox.

Ingestion transmission

Transmission through contaminated food or water.

Intestinal infections transmitted by ingestion

Cholera, dysentery, diarrheagenic E. coli, intestinal parasitic infections, and viral gastroenteritis.

Extraintestinal infections associated with ingestion

Salmonella Typhi, hepatitis A and E, poliovirus, hydatid disease, and cysticercosis.

Inoculation transmission

Direct introduction of a pathogen into skin or tissue, such as through an animal bite or infected wound.

Examples of inoculation transmission

Rabies through animal bites and tetanus through infected wounds.

Blood-borne transmission

Spread through infected blood, transfusions, or shared needles.

Examples of blood-borne infections

HIV and hepatitis B and C.

Vertical transmission

Transmission from mother to child across the placenta, as listed in the lecture.

Examples of infections associated with vertical transmission

Group B Streptococcus, Neisseria gonorrhoeae, Chlamydia, and HIV.

Vector-borne transmission

Transmission of pathogens by arthropods such as mosquitoes, flies, ticks, mites, and lice.

Mechanical vector

Carries a pathogen on its body without becoming infected.

Biological vector

Becomes infected and carries a pathogen that multiplies within the vector before transmission.

Example of biological vector transmission

Malaria.

Bacterial adherence

The bacterial glycocalyx helps bacteria adhere to surfaces such as skin and fomites.

Portals of entry

Openings such as the mouth, nose, urethra, genital tract, ears, and eyes, as well as cuts or breaks in the skin.

Factors affecting host susceptibility

Age, genetics, general health, exercise, nutrition, hygiene, immunocompromise, pathogen virulence, environmental factors, and pregnancy.

Types of pathogens

Viruses, bacteria, fungi, protists, and prions.

Virulence

The ability of a pathogen to cause disease through colonization, invasion, and damage to host tissues.

Virulence factors

Structures, structural change (in vessels) or substances that help pathogens invade tissues, damage the host, or evade the immune system.

Major virulence factors

Toxins, tissue-degrading enzymes, capsules, and changes in surface antigens.

Collagenase

An enzyme released by some bacteria, including Clostridium perfringens, that degrades collagen and helps tissue invasion.

Pro-inflammatory cytokines

Can increase blood vessel permeability, helping some bacteria gain access to tissues.

Helicobacter pylori urease

An enzyme that helps disrupt the protective mucus lining of the stomach, as described in the lecture.

Hyaluronidase

An enzyme that breaks down hyaluronan connecting cells, facilitating pathogen entry into tissues.

Botulism toxin

Reduces or prevents muscle contraction.

Tetanus toxin

Triggers sustained muscle contractions.

Bacterial capsules

Can prevent pathogens from being captured by phagocytic cells.

Antigenic variation

Changes in pathogen surface antigens that can help the pathogen evade immune recognition.

Seasonality as an environmental factor

Influenza and meningitis are more common in winter; malaria increases during rainy seasons when mosquitoes breed.

Environmental disinfectant resistance

Some organisms, including Pseudomonas, Acinetobacter, and Klebsiella, can persist in hospital environments and resist disinfectants.

Soil and pathogen survival

Damp sandy soil can support helminths (parasitic worms).

Moisture and pathogen survival

Microorganisms can survive in droplets.

Reservoir

A habitat in which a pathogen lives, grows, and multiplies before being transmitted to a host.

Types of reservoirs

People, animals, soil, water, and food.

Tetanus reservoir

Soil.

Hookworm reservoir

The lecture identifies humans as the reservoir and soil contaminated with hookworm larvae as the source of infection.

Typhoid reservoir

A case or carrier; contaminated food and water can serve as sources of transmission.

Human case

A patient who has the disease.

Carrier

A person who harbours an infectious agent without symptoms and can shed it through contact, air, or secretions.

Temporary carrier

Sheds an infectious organism for less than six months.

Incubatory carrier

Sheds a pathogen during the incubation period before symptoms appear.

Examples of incubatory carriers

Measles, mumps, polio, diphtheria, pertussis, hepatitis B, and influenza.

Healthy carrier

Harbours and sheds a pathogen without ever showing overt disease.

Examples of healthy carriers

Polio, cholera, salmonellosis, diphtheria, and meningococcal meningitis.

Convalescent carrier

Has recovered from disease but continues to harbour and shed the pathogen.

Chronic carrier

Sheds an infectious organism for an indefinite period.

Examples of chronic carriage

Hepatitis B, typhoid fever, malaria, and gonorrhoea, as listed in the lecture.

Types of human cases

Primary cases, index cases, secondary cases, clinical cases, subclinical cases, and latent infection cases.

Clinical case

A case with recognizable disease manifestations; severity or presentation may be mild, severe, typical, or atypical.

Subclinical case

An infection that does not produce obvious clinical manifestations.

Latent infection case

An infection in which the pathogen remains latent rather than producing continuous active disease.

Portals of exit

Pathways through which pathogens leave a host, including coughing, sneezing, body fluids, feces, vomit, skin, and open wounds.

Host susceptibility and previous exposure

Prior exposure may provide immunity; compromised immunity increases susceptibility.

Prophylaxis

Preventive measures used to reduce the risk of infection or disease.

Factors affecting infection risk

General health, nutrition, immune response, and pathogen virulence.

Principles of transmission control

Vaccination, prevention of exposure, prophylaxis, protection of susceptible people, prevention of disease progression, treatment of cases, and interruption of transmission.

Benefits of treating infectious disease

Can prevent death and disability and reduce further transmission.

Methods of interrupting transmission

Environmental controls and social or behavioural measures.

Outbreak

A sudden rise in cases within a geographic area.

Epidemic

Disease occurrence at a much higher rate than usual in a region.

Pandemic

An infection that spreads rapidly across large areas of the world.

Endemic disease

A disease that persists at a usually low level in a particular region.

Examples of endemic diseases in India

The lecture lists typhoid fever, cholera, filariasis, and malaria.

Hyperendemic disease

A disease that persists at a high level in a geographic area.

Hyperendemic examples in India

The lecture lists hepatitis A and hepatitis E.

Sporadic disease occurrence

Infections that occur at irregular intervals or in isolated locations.

Purposes of disease investigation

Establish a diagnosis, identify the agent and source, describe cases by person, place, and time, determine transmission mode, and identify susceptible populations.

Host

A person or animal that supports an infectious agent under natural conditions.

Definitive (primary) host

a host in which a parasite reaches maturity and reproduces if possible

Obligate host

A host required for a pathogen’s life cycle.

Intermediate host

organism that has immature pathogen or parasite, but does not reach sexual maturity

Transport host

carries immature parasite without it growing or multiplying

Zoonosis

An infection transmissible from vertebrate animals to humans.

Examples of zoonoses

Rabies, plague, and bovine tuberculosis.

Epizootic

An outbreak or epidemic of disease in an animal population.

Enzootic

A disease that is endemic in an animal population; bovine tuberculosis is an example.

Nosocomial infection

An infection acquired in a hospital or healthcare setting.

Examples of nosocomial infections

Surgical wound infections, hepatitis B, and urinary tract infections.

Incubation period

The time from infection to the appearance of symptoms.

Latent period

The time from infection until the person becomes infectious.