Chapter 14 Disease and Epidemiology

Microbiome

all microbes in a human body and their genetic material

Human Microbiome (Microbiota)

Microbiome, upper respiratory, skin, large intestines “gut microbiota” , parts of urinary and reproductive systems

Human Microbiome Project

large scale effort to research and map typical ranges of microbial diversity.

Commensalism

one organism benefits and the other is unaffected. We provide “food & lodging”, no direct benefit to us from the

microbes.

Mutualism

both organisms benefit

We get something in return: protection from pathogenic

bacteria, vitamins, immune system training

Parasitism

one organism is benefited at the expense of the other (bacterial pathogens). Some microbes are opportunistic (an organism don’t cause disease unless appropriate condition exists or it gets into the “wrong” place)

Competition exists where?

between microbes (microbial antagonism)

Probiotics

live microbes are applied to or are ingested into the body

Antibiotics

chemicals applied to or are ingested to kill particular bacteria

If the presence of a certain bacterial populations protects an individual from Clostridium difficile infection, what kind of relationship these bacterial populations have with a host?

Mutualism

Getting Your Microbiome

Usually germ-free in uterus

Breastfeeding promotes “milk-based” infant microbiota

Bifidobacterium and Lactobacillus dominate

Classifying Diseases

-Communicable disease (infectious)

-Noncommunicable disease

Noncommunicable disease

a disease that is not transmitted from one host to another

- caused by a microbe (common source epidemics)

-> majority are not due to microbes (cancer, diabetes, heart diseases)

Communicable disease (infectious)

a disease that is spread from one host to another through a pathogen

Pathogen

a disease causing microbe

Pathogenicity

how a disease develops, ability of a microbe to cause a disease

Infection

growth of microbes in or on the body (where they are not usually present)

Robert Koch

1st to definitely show that microorganisms actually caused diseases

Koch’s Postulates

-relates a specific microbe to a specific disease-

- this is how one shows that a microbe is a cause-

1. The same organisms must be found in all cases of a given

disease.

2. The organism must be isolated and grown in pure culture.

3. The isolated organism must reproduce the same disease when

inoculated into a healthy susceptible animal.

4. The original organism must again be isolated from the

experimentally infected animal.

-> 2,3,4 tests for isolation, demonstration of the same disease

-> There are some exceptions to Koch’s postulates

(multiple diseases, growth in the culture)

Classifying Diseases

-symptom

-sign

-syndrome 

Symptom

how the patient feels – any changes? – subjective and can be vague (pain, body aches)

Sign

objective, concrete view, direct observation, can be measured (body temperature, oxygen levels, rush)

Syndrome

A specific group of symptoms or signs that always accompany a specific disease

Incidence

fraction of a population that gets a disease during a specific time; rate of occurrence of new cases

Prevalence

fraction of a population having a specific disease at a given time; total number of cases of disease within population

Local Infection (Extent of Host Involvement)

confined to an area of the body, e.g. pimples

Systemic Infection (can come from local infection)

microbes or their products (e.g. toxins) have spread throughout the body; also disseminated

Epidemiology

study of transmission and incidence

Morbidity

incidence of a specific disease

Mortality

deaths from a disease

Endemic

low, constant numbers of a disease in a particular geographical area

Epidemic

high numbers over a short duration, in a specific region

Pandemic

worldwide epidemic, e.g. influenza, SARS-CoV-2

Acute disease

rapidly developing, short duration, e.g. influenza

Latent infection

microbe inactivated for period of time, can be triggered to reactivate, e.g. cold sores, chicken pox – shingles, HIV – AIDS

Chronic disease

long duration (usually more than a year), e.g. tuberculosis; “chronic” often refers to non-infectious disease (diabetes)

Predisposing factors

make the body more susceptible to disease, like:

• Climate & weather / season

• Fatigue

• Age

• Lifestyle

• Chemotherapy and treatments affecting the immune system

• Short urethra in females

Risk factors

of getting a disease include travelling to areas where disease is prevalent, interacting with infectious people and other.

The stages of an infectious disease

• Incubation

• Prodromal period

• Period of illness

• Period of decline

• Period of convalescence

Incubation

The period of incubation is the time occurring between the start of an infection and the appearance of symptoms

Prodromal period

It is the beginning of appearance of symptoms (often not specific). Acquired immunity (vaccination and previous exposure) may help to stop a disease.

Period of decline

Pathogen replication is brought under control either by host immune response or through outside intervention

Period of convalescence

The body regains its pre-illness strength

Case study: Tuberculosis

Caused by: Mycobacterium tuberculosis

World:

• 8,331: reported TB cases in the United States in 2022 (a rate

of 2.5 cases per 100,000 persons)

• 60: jurisdictions (e.g., states, cities, U.S. territories, and

affiliated areas) that report TB data to CDC

Is prevalence or incidence reported for tuberculosis above?

What is Tuberculosis transmission?

airborne

Different scenarios of TB

a) A person is infected, but the body successfully contains the spread (latent TB). This person cannot spread TB.

Up to 13 million: estimated number of people in the United States

living with latent TB infection

b) A person is infected and develops an active form of tuberculosis (TB disease). This person can spread TB. In 5-10% of cases latent can become active.

Risk factors for getting infected

• Exposure to TB infectious person (sick with TB disease).

• Being born or often travelling to countries with higher TB

transmission rates than the US: Mexico, China, India, Philippines.

• Populations of people, healthcare workers and other professions who live or work at places with high TB transmission rates

Reservoir

– habitat of the infectious agent

– environment, living organism

Spread of Microbes

Reservoir:

A) Humans

Infectious person

Carrier – an individual who is usually asymptomatic but is carrying the microbe; They may have a latent infection, e.g. HIV, Typhoid Fever –bacteria survive in the gall bladder; May or may not be spreading the disease (depends on the treatment or the disease)

B) Animal Reservoirs

(Rabies virus – dog, Salmonella – turtles)

C) Nonliving Reservoirs

(soil, water, food, botulism)

Direct

person to person

Contact

e.g. sexually transmitted diseases or mother-child

infectious droplets (aerosols)