Pathology →

• Etiology - cause of disease

• Pathogenesis - manner in which the disease develops

• Bodily Changes - structural/functional changes due to disease

Infection vs. Disease

Infection is what happens before a disease. Infection does not always lead to symptoms.

Disease is the outcome of infection. This is what happens when symptoms of an infection appear.

Virulence vs. Pathogenicity vs. Infectivity

Virulence - relates to how serious the pathogen is

• EX: flu compared to ebola

Pathogenicity - refers to the ability to cause diseases

Infectivity - how transmissible it is

Infectious Disease Cycle

Starts with a pathogen which will inhabit a reservoir (source of pathogen).

→ where the pathogen naturally resides. EX: Meningitis - other humans

the pathogen is then transmitted from the reservoir and to a susceptible host. It enters the host, goes through the host barriers, invades the host, before it can damage the host → leads to pathology

Whether these steps happen is dependent on microbial pathogenesis which relates to virulence factors.

Virulence factors - the features a pathogen has that enables it to cause disease

• virulence of a pathogen depends on this

Human Microbial Flora

• The human microbial flora are beneficial to us

• The human body is occupied by millions of bacteria; collectively, they are termed the normal microbiota

• These bacteria residing in your body prevents other organisms from taking up resident in and on your body.

  • Acquisition of flora: you acquire it from birth from the birth canal before it grows from eating, breathing, etc as you grow.

  • Microbes colonize non-sterile areas: those exposed to external environment (skin, oral & nasal cavities, lungs, etc.)

  • The microbiota species varies throughout life: transient microbiota

  • Presence of microbes in sterile areas signal infection

Transient microbiota: temporary microbiota that reside in the host’s body

Microenvironments - different parts of our body have different conditions such as pH levels, osmolarity, oxygen levels, etc. Different microbiota inhabit these different microenvironments

Normal microbiota → exhibit microbial antagonism or competitive exclusion

• Makes it difficult for other bacteria/organisms to establish themselves because they’re competing against already established normal microbiota

• Prevents colonization of pathogenic microbes

  • Compete for nutrients, space, produce agents, modify their microenvironment

  • Balance is upset → disease can result

Symbiotic relationship between host : normal microbiota

• Endosymbiosis - our bacteria are intimately associated with us

• Commensalism: +/0

• Mutualism: +/+

• Parasitism: +/-

+ = benefit to member, - = detrimental to member, is harmed, 0 = neither beneficial nor harmful

Normally benign bacteria can become opportunistic pathogens and cause disease when host defenses are compromised

• Asymptomatic carriers

Opportunistic Pathogens vs Primary pathogens:

• Primary pathogens - the purpose of this pathogen is to cause disease

• Opportunistic pathogens - doesn’t cause disease all the time. Cause disease when host defenses are compromised

Koch’s Postulates:

• Same pathogen present in all cases

• isolation of pathogen from diseased host → obtain pure culture

• W the pure culture isolate → causes disease in susceptible host

• Re-isolate pathogen from 2nd host; can you re-isolate the pathogen? Confirm it’s the same as from the first host

Exceptions →

• Healthy individuals can be carriers, not just diseased

• Culturability of the microbe → you cannot always create a pure culture of certain pathogens

• Similarity of symptoms of disease & same disease caused by multiple pathogens → One disease is not caused by one pathogen. Multiple pathogens can cause one same disease.

• Same pathogen causing different disease conditions → one pathogen doesn’t cause one disease. One pathogen can cause multiple different diseases.

• Viral agents → use tissue culture; some have no suitable animal model/host; latent types (you don’t see symptoms until later on)

Classifying Infectious Diseases:

• A disease alters body structure & functions in a specific ways

Symptoms - Subjective; not measurable such as headache, dry throat, etc.

Signs - Objective; measurable such as fever, rash, swelling, etc

Syndrome - a group of signs/symptoms that occur together and characterize a particular abnormality or conditions

Communicable disease - can directly/indirectly spread from host to host

• Contagious disease - easily spread from person to person (plague, cold, pink eye, etc)

Non-communicable disease - do not spread from host to host (tetanus)

Occurence of Disease

Incident - number of people in a population who develop a disease during a particular time period (indicates spread of disease or how fast a disease is spreading)

Prevalence - number of people in a population who develop a disease at a specified time, regardless of when it first appeared (old & new cases; indicates seriousness of the disease)

• Together enable: estimation of the range of a disease’s occurrence and its tendency to affect certain groups

Expected Prevalence - predicted prevalence based on previous data and patterns

Sporadic - if only a few cases happen in a certain region at a certain time; occurs infrequently

Endemic - normally and continuously there at a fairly stable rate; always present at around a stable rate

Epidemic - If a disease occurs at a region at a rate significantly higher than what would normally be expected; rapid development of disease in a specific area

Pandemic - A disease that occurs at multiple regions at the same time at a rate significantly higher than what would normally be expected; an endemic that happens at multiple places/continents at the same time; worldwide epidemic

Severity or Duration of a Disease:

• Acute disease - symptoms develop rapidly; flu

• Chronic disease - disease develops slowly; tuberculosis

• Subacute disease - symptoms between acute and chronic

• Latent disease - disease with a period of no symptoms when a causative agent is inactive

Herd immunity - immune individuals act as barriers to the spread of infection; advantage of vaccination - those not immune are protected (only for communicable diseases)

• RO value - the average number of people who will contract a contagious disease from one person with that disease

Infection classification based on the extent of infection in the body

• Local: limited, confined to a specific area

• Systemic: spreads throughout body via circulatory/lymphatic system

• Focal: spreads from a local infection to specific parts of the body; teeth, tonsils, sinuses

• Sepsis: inflammatory condition due to presence of toxin/bacteria in blood - spread from focus of infection; septicemia an example of this

The state of host resistance affects extent of infection:

• Primary infection - causes initial illness (acute)

  • Leaves you susceptible to other illnesses because your immune system is compromised

• Secondary infection - caused by opportunistic pathogen following primary infection

• Subclinical (inapparent) infection - no noticeable signs or symptoms of illness

Predisposing Factors

Makes the body more susceptible to disease:

• Females vs. males, inherited traits, such as the sickle cell gene

• Climate and weather, fatigue, age, lifestyle, chemotherapy

Development of Disease:

• Incubation - no symptoms yet, pathogens multiplying

• Prodromal - mild signs of symptoms: tiredness, mild headache, etc

• Period of illness - Where most severe signs and symptoms are; body is fighting off infection; fever

• Period of decline - signs and symptoms lessen

• Period of convalescence - recovery

Incubation time varies on the pathogen

Prodromal period doesn’t necessarily characterize what disease you have. Period of illness is where you get more obvious symptoms of a disease and you can get your diagnosis

Throughout the whole period, until you reach the end of the period of convalescence, you are a carrier for that disease.

Spread of Infection

reservoir of Infection - source of the disease microbes (where we would normally find it)

• Human: show symptoms of disease or do not (carriers); HIV; Hepatitis; STDs; Meningitis

  • Carriers - have in apparent infections or have latent diseases

• Animal: Zoonotic diseases; Zoonoses (diseases that start in animals and make their way to humans

• Non-living: soil, water (tetanus

Infection cycle - the route an organism takes from one individual to another

Transmission of Disease: via contact, vehicles, & vectors

• Contact (direct) - physical contact between source & host

• Contact (indirect) - transmission via inanimate object → fomites (objects that carry the infections)

• Contact (droplet) - travels through the air, short distance; sneezing, coughing

  • Travels < 1 meter

Vehicle - transmits disease via water, food, or air; also blood and drugs

Waterborne - contaminated (fecal) water, treatment system compromised

Airborne - Dust particles or pet danders

Foodborne - contaminated food, storage @ improper temperature, undercooked food, unsanitary handling of food

Fecal-oral transmission - can occur as a result waterborne and foodborne transmission. Ingested contaminated food or water

Congenital transmission - mother → child; diseases passed on from mother to child through childbirth

Vectors - insects typically

• Mechanical transmission: passive via body parts

  • Fly sits in trash can → pathogen on fly → fly lands on you → you now have pathogen

• Biological transmission: involves parasite life cycle

  • Malaria, west nile, zeka, mosquitos bringing pathogen

  • Biting → blood gets infected from bite

Health care-associated infections (HAIs)

Are acquired as a result of receiving treatment at a healthcare facility; affect 4-5% of all hospital patients; such infections result from different interactions:

• Microorganisms in hospital environment

  • Opportunistic pathogens present the danger to hospital patients. Also drug resistant type are prevalent

• Compromised host

  • Resistance to infection is impaired by disease, therapy, or burns. Host compromised due to: broken skin or mucous membranes & suppressed immune system

• Chain of transmission

  • Routes of transmission: direct contact from staff or other patients; indirect (fomites, airborne)

→ Nosocomial infections or HAIs

Sources of infection could be devices to assist patients

(Do i need to know the diseases?)

Control of HAIs

Universal precautions - to reduce the transmission of microbes in health care and residential settings

→ Standard precautions - basic, minimum; applied to all levels

• Aseptic techniques, handling contaminated materials, frequent handwashing, use of isolation rooms & wards, use of PPE, educated staff

→ Transmission-based precautions - to supplement standard precautions in individuals with know/suspected infections that are highly transmissible

• Contact precautions, droplet precautions (short distance), Airborne precautions (long distance)

Emerging Infectious Diseases

EIDs - new or changing, show an increase in incidence in the recent past, or show a potential to increase in the near future

• Can be caused by a virus, bacterium, fungus, protozoan, or a helminth

• 75% of emerging infectious diseases are zoonotic, mainly of viral origin, and are likely to be vector borne

Criteria for EIDs:

• distinctive disease symptoms

• improved diagnostic techniques allow ID of new pathogen

  • local disease becomes widespread

  • rare disease becomes more common

  • mild disease becomes more severe

Factors contributing to EIDs:

• Genetic recombination/evolution

  • Zoonotics

• misuse of antibiotics/pesticides

• climate change

• increased human transportation

• ecological changes

• public health failures

Epidemiology

Epidemiology - The study of where and when diseases occur and how they are transmitted in populations. Important for:

• Determining the etiology of a disease

• Identifying other important factors concerning the spread of disease

  • Who’s being impacted by the disease the most?

  • demographic data, socioeconomic status, common histories, etc

• Developing methods for controlling a disease

  • control of reservoirs, food inspections, sewage disposal, etc

• Assembling data and graphs to outline incidence of disease

  • Determining disease frequency; test effectiveness of disease control (vaccination), when does it occur the most

Epidemiological types

Descriptive epidemiology - collection and analysis of data describing the occurrence of disease

• John Snow: mapped occurrence of cholera cases in London; source traced to Broad street water pump

Analytical epidemiology - analyzes a particular disease to determine its probable cause

• Florence Nightingale: used medical statistics to demonstrate high mortality rate due to disease (typhus) among soldiers during warfare

Experimental epidemiology - involves a hypothesis and controlled experiments

• Semmelweis: hand washing decreased incidence of childbirth fever (puerperal sepsis)

• Clinical trial - test and control group

Case reporting - health care working report specified disease to local, state, and national offices (AIDS, meningitis, STD’s, tetanus)

• Establishes the chain of transmission

National notifiable infectious diseases - physicians are required to report occurrence

Centers of Disease Control and Prevention (CDC)

• Collects and analyzes epidemiology information in the US

• Publishes Morbidity and Mortality Weekly Report

  • Morbidity - incidents of a specific notifiable disease

  • Mortality - deaths from notifiable diseases

  • Morbidity rate & Mortality rate