7. Infections of Populations & Barriers to Infection

Pathogenesis

process of producing a disease

Nosocomial infection

infection acquired inside a hospital or clinical setting.

Iatrogenic infection

Results directly from a medical procedure or intervention

Endogenous infection / Proviral integration

when viral DNA becomes part of the host genome

Vertical transmission

Occurs during pregnancy, birth, or breastfeed

Infection spread among individuals of the same species

Transmission between members of the same species

Epidemiology

Study and analysis of distribution and determinants of disease

Case-fatality ratio

deaths within a population of infected individuals (typically expressed as a

percentage)

Incidence

number of new cases of a disease in a population over a specific period of time.

Morbidity

state of being diseased or the burden of illness in a population.

Mortality

number of deaths in a population due to a disease.

What is R₀?

number of secondary infections caused by ONE infected individual in a fully susceptible population.
How contagious is this

R₀ < 1

Each infected person infects less than one new person on average —> NO EPIDEMIC

R₀ > 1

Each infected person infects more than one new person —> possible epidemic

R₀ ≫ 1 (much greater than 1)

Extremely efficient transmission —> epidemic is certain

Aedes aegypti

key mosquito vector that maintains and spreads arboviruses among humans.

Why is Yellow Fever Virus historically significant?

first identified human virus.

How is Yellow Fever Virus transmitted?

Through mosquito vectors, primarily Aedes aegypti.

First view of west nile virus in the US

was in 1999, when it was identified in New York City.

west nile virus by 2003

virtually all of US (and much of Canada)

What mosquito species originally transmitted Chikungunya virus?

Aedes aegypti

What evolutionary change allowed Chikungunya virus to expand into new regions?

A point mutation in the viral genome.

What was the effect of this point mutation on viral transmission?

enabled efficient transmission by Aedes albopictus.

Why is transmission by Aedes albopictus significant for global spread?

Aedes albopictus lives in temperate climates, allowing Chikungunya to spread beyond tropical regions.

Spread into new human hosts (zoonoses) are usually

dead-end infections

Social interactions, individual differences among prospective hosts, group dynamics and

behaviors, geography, and weather all influence how

efficiently a virus can establish infection

within a population

The regional occurrence of viral infections may be due to the

restrictions of a vector or animal

reservoir to a limited geographic area

Susceptibility to infection and susceptibility to disease

are independent

What does it mean for a cell to be “susceptible”?

expresses the receptor needed for viral entry.

What does it mean for a cell to be “permissive”?

contains the internal factors needed for viral replication.

Why must local antiviral defenses be overcome for infection to succeed?

innate immunity can block replication at the earliest steps, preventing the virus from establishing itself.

Why is the amount of virus important for infection?

minimum infectious dose is required; too little virus cannot overcome host defenses or reach enough target cells.

Name four major virus families that enter the body through breaks in the skin

Flaviviruses (WNV, YFV, dengue, Zika), Herpesviruses (HSV, EBV, CMV), Poxviruses, Papillomaviruses (HPV), Rabies virus.

Why is intact skin generally an effective barrier to viral infection?

It is thick, keratinized, and lacks receptors for most viruses.

What is the primary physical barrier in the respiratory tract that prevents viral infection?

Cilia, which move debris and trapped pathogens out of the airway.

Why is the respiratory tract a common site of viral entry?

large surface area, is constantly exposed to the environment, and contains permissive epithelial cells.

Name five viruses that enter through the alimentary (GI) tract.

Enterovirus, Rotavirus, Norovirus, Adenovirus, Coronavirus.

Name four viruses that enter through the urogenital tract.

HIV, HBV, HSV, HPV.

Name five viruses that can enter the body through the eye (conjunctiva).

RSV, HSV, Adenovirus, Influenza virus, Enterovirus.

Sites of Viral Entry — Fetus

TORCH

What does the acronym TORCH stand for?

• Toxoplasma gondii

• Other (syphilis, VZV, parvovirus B19, Zika, etc.)

• Rubella

• Cytomegalovirus (CMV)

• Herpes simplex virus (HSV)

What two major categories of barriers must a virus overcome to spread within a host?

Physical barriers and immune barriers.

Apical release

facilitates virus extracellular spread

Basolateral release

provides access to underlying tissues,

may facilitate systemic spread (often by viremia)

Viremia

presence of infectious virus particles in the blood

Diapedesis

The movement of leukocytes across the endothelial barrier, either paracellularly or transcellularly

Paracellular diapedesis

leukocytes pass between endothelial cells through cell junction

transcellular diapedesis

leukocytes pass through an endothelial cell body

How does mousepox disseminate from the primary site of infection?

Via hematogenous spread (viremia).

What type of inclusion bodies are characteristic of poxvirus‑infected epithelial cells?

Intracytoplasmic inclusion bodies.

Neurotropic

can infect neural cells

Neuroinvasive

enters CNS after infection at peripheral site

Neurovirulent

causes disease of nervous tissue

Which virus is both highly neuroinvasive and highly neurovirulent?

Rabies

Mumps

high neuroinvasiveness, low neurovirulence

HSV

low neuroinvasiveness, high neurovirulence

Viral shedding

the release of infectious virus from an infected host into the environment.