How Viruses are Transmitted & Cause Disease Flashcards
Incidence
Number of new infections over a period of time
Prevalence
Total number of people infected in a population, new and old, over a period of time
Endemic
Normal number of cases in a population. The number can be high, low, and even seasonal
Epidemic
An increase in the average number of cases in an area
Pandemic
An epidemic that has spread to several countries or continents
R0-Reproductive number
On average, how many people one infected person will infect in a susceptible population
Describe effective reproductive number (Re or Rt)
Effective reproductive number (Re or Rt) is spread in a population where not everyone is susceptible. R0 is fixed, but the Re can change with changing variables
(social distancing, mask-wearing, immunity by infection, or vaccination). When people talk about getting R below 1, they are talking about Re.
An infectious disease transmitted from one source to another
Communicable
- Person to person
- Animal to person
- Fomite to person (phone, pen, shopping cart handle)
Derived from contact. Is considered a very communicable disease spread by contact or proximity to an infected person and is transmitted by:
Contagious
- Respiratory droplets
- Fecal/oral
- Contact with skin or abrasion (less common)
ex: Ebola, gastroenteritis, respiratory infections
Communicable but not by casual contact
Non-contagious
Non-contagious infections are transmitted through:
1) Sexual contact - semen, vaginal fluid, or genital lesions
2) Parenteral/Injection - shared needles
3) Congenital - transplacental from mother to fetus
4) Perinatal - pregnancy (after 24wks), during or shortly after birth
5) Arboviral - mosquito, tick
6) Zoonotic - transfer from infected non-human animals to humans
ex: bloodborne pathogens, HIV, HBV, HCV, sexually transmitted infections, HSV-1, HSV-2, HPV, arboviruses, Zika, dengue, yellow fever
Describe the viruses that are transmitted in the Winter/Spring
This is the season for many viruses that are enveloped and transmitted through the respiratory route and many gastroenteric viruses
Describe the viruses that are transmitted in the Summer/early fall
This is peak season for arboviruses and picornaviruses
Incubation period
The time from infection until symptoms appear is the incubation period
Latent period
In the latent period, the person is infected, but no virus is shed from the body. It is the time from infection until you become infectious and capable of spreading the disease to others
Infectious phase
The infectious phase is when the infected individual can spread the disease to others. Notice that the person can be infectious before symptoms appear. Depending on the virus, infectiousness stops either before symptoms clear, when symptoms cease, or a person remains infectious for some time after symptoms stop
Prodromal phase
The prodromal phase refers to the generalized symptoms that can occur in many viral infections before specific symptoms arise that might aid in diagnosis.
For example, in measles, the prodromal phase is the time the person feels sick before the measles-defining rash
Prodromal phase
The prodromal phase refers to the generalized symptoms that can occur in many viral infections before specific symptoms arise that might aid in diagnosis.
For example, in measles, the prodromal phase is the time the person feels sick before the measles-defining rash
What is the iceberg concept of infection?
In most infections, the majority of people infected are asymptomatically or subclinically infected. They have no symptoms but can usually spread disease (but usually not as well as people who have symptoms)
List the 3 factors that determine the severity of infection
- Age
- Immune status
- Genetics of the host
Describe why age is a factor in infection severity
-Infections in children can be worse or sometimes better than in the general population
-Infections in people over 65 are usually worse. Their immune system doesn’t work as well, and they tend to have other health problems that can make some infections worse
Describe why immune status is a factor in infection severity
-Immunocompromised patients have a much worse outcome with infections that can be relatively benign to the general population
-Prior exposure to the infectious agent, either by vaccination or previous infection, can prevent infection or lead to milder symptoms
-The general health of the host can be a factor in disease severity
Describe why genetics of the host is a factor in infection severity
Sometimes, genetic determinates make certain people more or less susceptible to disease.
Describe human to human host transmission
This is the most common form of transmission and has no animal reservoir and can be:
1. Horizontal - most common, from person to person
2. Vertical - from mother to infant (either in utero or during birth)
Describe Zoonotic host transmission
Spread of disease from a non-human reservoir to a human host. It can be:
1. Indirect with an insect intermediary called the vector
2. Direct from the infected non-human animal to the human
Describe Localized infections
-Doesn’t spread throughout the body, no viremia (virus present in the blood). Some but not all infections of the respiratory tract/GI tract, eye, skin
-Shorter incubation times, usually days
-IgA is usually important
-Not always lifelong immunity, sometimes shorter mucosal immunity
Describe Generalized/Systemic infections
-Spreads through the body, via blood (viremia) or nervous system
-Longer incubation times, weeks to months
-IgG is more important
-Usually lifelong immunity
