How Viruses are Transmitted & Cause Disease

Question 1

Incidence

Answer 1

Number of new infections over a period of time

Question 2

Prevalence

Answer 2

Total number of people infected in a population, new and old, over a period of time

Question 3

Endemic

Answer 3

Normal number of cases in a population. The number can be high, low, and even seasonal

Question 4

Epidemic

Answer 4

An increase in the average number of cases in an area

Question 5

Pandemic

Answer 5

An epidemic that has spread to several countries or continents

Question 6

R0-Reproductive number

Answer 6

On average, how many people one infected person will infect in a susceptible population

Question 7

Describe effective reproductive number (Re or Rt)

Answer 7

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.

Question 8

An infectious disease transmitted from one source to another

Answer 8

Communicable

1. Person to person
2. Animal to person
3. Fomite to person (phone, pen, shopping cart handle)

Question 9

Derived from contact. Is considered a very communicable disease spread by contact or proximity to an infected person and is transmitted by:

Answer 9

Contagious

1. Respiratory droplets
2. Fecal/oral
3. Contact with skin or abrasion (less common)

ex: Ebola, gastroenteritis, respiratory infections

Question 10

Communicable but not by casual contact

Answer 10

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

Question 11

Describe the viruses that are transmitted in the Winter/Spring

Answer 11

This is the season for many viruses that are enveloped and transmitted through the respiratory route and many gastroenteric viruses

Question 12

Describe the viruses that are transmitted in the Summer/early fall

Answer 12

This is peak season for arboviruses and picornaviruses

Question 13

Incubation period

Answer 13

The time from infection until symptoms appear is the incubation period

Question 14

Latent period

Answer 14

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

Question 15

Infectious phase

Answer 15

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

Question 16

Prodromal phase

Answer 16

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

Question 17

Prodromal phase

Answer 17

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

Question 18

What is the iceberg concept of infection?

Answer 18

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)

Question 19

List the 3 factors that determine the severity of infection

Answer 19

1. Age
2. Immune status
3. Genetics of the host

Question 20

Describe why age is a factor in infection severity

Answer 20

-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

Question 21

Describe why immune status is a factor in infection severity

Answer 21

-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

Question 22

Describe why genetics of the host is a factor in infection severity

Answer 22

Sometimes, genetic determinates make certain people more or less susceptible to disease.

Question 23

Describe human to human host transmission

Answer 23

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)

Question 24

Describe Zoonotic host transmission

Answer 24

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

Question 25

Describe Localized infections

Answer 25

-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

Question 26

Describe Generalized/Systemic infections

Answer 26

-Spreads through the body, via blood (viremia) or nervous system
-Longer incubation times, weeks to months
-IgG is more important
-Usually lifelong immunity