Chapter 2- Dynamics of Disease Transmission

Question 1

Epidemiologic triad

Answer 1

3 factors interact to cause disease
1. Host (person)
2. Infectious agent
3. Environment
Sometimes, a vector is also involved

Question 2

At the host level, what 3 factors interact to cause disease?

Answer 2

Genetics, behavior, and environment. The proportion of the role of each of these factors differs by disease

Question 3

Human susceptibility

Answer 3

A person must be susceptible to a disease to develop it. Susceptibility is determined by many factors, including genetics, behavior, and nutritional and immunological characteristics. A person’s immune status is also determined by many different factors

Question 4

Diseases can be transmitted (2)

Answer 4

Directly or indirectly

Question 5

Direct transmission

Answer 5

When disease is transmitted from person to person through direct contact (like sexual contact)

Question 6

Indirect transmission

Answer 6

When transmission occurs through a common vehicle like contaminated air or water supply, or by a vector like a mosquito

Question 7

Body surfaces as sites of microbial infection and shedding (7)

Answer 7

There are multiple body surfaces that microorganisms can shed from, or that microorganisms or toxins can enter through
1. Mouth
2. Anus
3. Alimentary tract
4. Respiratory tract
5. Urogenital tract
6. Conjunctiva
7. Skin- absorption or through a scratch, insect bites may also penetrate the skin

Question 8

Iceberg concept of disease

Answer 8

Most of an iceberg is under water, with only the tip being visible. With disease, only clinical illness is readily apparent (the “tip of the iceberg”). This is because infections without clinical illness occur, where the person feels fine but is still contagious. Additionally, people with chronic diseases may have the disease for a long period of time without exhibiting symptoms and may only be diagnosed when they experience a complication

Question 9

Spectrum of severity

Answer 9

Some diseases are very severe and rarely inapparent (like measles), while others are likely to be inapparent and generally wouldn’t be diagnosed until symptoms develop (tuberculosis). Rabies is another severe disease that is almost always fatal without intervention. Severity seems to be related to the virulence of the organism and to the site in the body where the organism multiplies

Question 10

Virulence

Answer 10

How efficient the organism is at producing disease

Question 11

Clinical disease

Answer 11

Characterized by signs and symptoms. Infectious disease transmission may be influenced by both clinical and nonclinical disease

Question 12

Nonclinical (inapparent) disease categories (4)

Answer 12

1. Preclinical disease
2. Subclinical disease
3. Persistent (chronic) disease
4. Latent disease

Question 13

Preclinical disease

Answer 13

Disease that is not yet clinically apparent but is destined to progress to clinical disease

Question 14

Subclinical disease

Answer 14

Disease that is not clinically apparent and is not destined to progress to clinical disease

Question 15

Persistent (chronic) disease

Answer 15

A person fails to clear the infection, and it persists for years or even for life

Question 16

Latent disease

Answer 16

An infection with no active multiplication of agent

Question 17

Carrier

Answer 17

An individual who harbors the organism but is not infected (does not have a serologic/antibody response) or has no evidence of clinical illness. This person can still infect others, although infectivity is generally lower. Carrier status can be of limited duration or can be chronic

Question 18

Endemic

Answer 18

The habitual presence of a disease within a given geographic area. It can also refer to the usual occurrence of a given disease within an area (the “background rate of disease”).

Question 19

Epidemic

Answer 19

Disease has increased in frequency in a geographic area above its usual rate

Question 20

Pandemic

Answer 20

A worldwide epidemic

Question 21

How do we know when we have an excess of infections over what would be expected?

Answer 21

Through ongoing surveillance, it might be possible to determine what the usual or expected level of disease could be

Question 22

Common-vehicle exposure

Answer 22

When all cases of an infection occur in people who have been exposed to the same thing (like all cases of food poisoning occurring in people who consumed the same contaminated food item)

Question 23

Single exposure

Answer 23

When people were only exposed to the vehicle of infection once (they were only served contaminated food one time)

Question 24

Multiple exposure

Answer 24

When people were exposed to the vehicle of infection multiple times (they were served contaminated food more than once)

Question 25

Periodic exposure

Answer 25

When the contaminated vehicle exposes people intermittently, like when a water supply is contaminated with sewage due to leaky pipes. The exposure is periodic if multiple exposures occur due to changing pressures in the water supply system, for example

Question 26

Continuous exposure

Answer 26

When the vehicle causes constant and persistent contamination, like if a constant leak of sewage leads to persistent water contamination

Question 27

Single exposure, common vehicle outbreaks

Answer 27

These outbreaks are generally explosive, with cases increasing suddenly and rapidly in the population. The cases are also limited to people who share the common exposure. Additionally, in a food borne outbreak, cases rarely occur in people who did not eat the food- people acquire the disease from a primary case who ate the food

Question 28

How do immunity and susceptibility determine the amount of disease in a population?

Answer 28

The amount of disease in a population depends on the balance between the number of people in that population who are susceptible and at risk for the disease and the number of people who are immune and therefore not at risk. Immunity can be acquired through previous infection or through vaccination. They could also be immune due to genetics. There is generally a balance between susceptibility and immunity in the population. If the balance of the population moves toward susceptibility, an outbreak is more likely to occur.

Question 29

Herd immunity

Answer 29

A decreased risk of disease among susceptible individuals by the presence/proximity to immune individuals. If a large percentage of the population is immune, the entire population will likely be protected. Lack of vaccine effectiveness at totally preventing disease and lack of vaccine coverage makes herd immunity difficult to achieve

Question 30

Why does herd immunity occur?

Answer 30

Disease spreads from one person to another in every community. When a certain amount of people in a population are immune, it is unlikely that an infected person will encounter a susceptible person to transmit the disease to

Question 31

Why is herd immunity so important?

Answer 31

When carrying out immunization programs, it might not be necessary to vaccinate 100% of the population. Highly effective protection can be achieved by vaccinating a large proportion of the population, and the rest of the population can be protected through herd immunity

Question 32

Which conditions must be met to achieve herd immunity?

Answer 32

1. The infectious agent must be restricted to a single host species where transmission occurs
2. Transmission must be relatively direct from one member of the host species to another. If the organism can exist and have a reservoir outside the human host, herd immunity can’t work because there are other mechanisms of infection
3. Infections must induce solid immunity. Partial immunity will not allow there to be a large proportion of immune individuals in the population

Question 33

When does herd immunity operate optimally?

Answer 33

When populations are constantly mixing together. If an infected person has the same probability of interacting with every other person in the population (“random mixing”), herd immunity works. However, if the infected person primarily interacts with susceptible people, they are likely to transmit the disease. Populations are never completely randomly mixed, since everyone tends to interact with their family and friends more frequently. However, the degree to which herd immunity is achieved depends on the extent to which the population approaches random mixing

Question 34

What percentage of a population must be immune for herd immunity to operate?

Answer 34

The percentage is different depending on the disease. Measles is highly contagious, so it has been estimated that 94% of the population must be immune to interrupt the chain of transmission

Question 35

Incubation period

Answer 35

The time between infection and the onset of clinical illness. During the incubation period, people do not exhibit any symptoms. In general, the incubation period is characteristic of the infectious agent

Question 36

Why doesn’t disease develop immediately at the time of infection?

Answer 36

The incubation period could reflect the time needed for the organism to replicate sufficiently until it reaches high enough numbers to cause disease. It could also relate to the site in the body where the organism replicates- does it replicate in the skin or deeper in the body? The dose of the infectious agent can influence the incubation period- a higher dose may shorten the incubation period

Question 37

What determines the period of isolation of an infected person?

Answer 37

The period of isolation is generally determined by the incubation period. When a person is clinically ill, they are obviously infectious. However, during the incubation period, a person can be infectious without exhibiting symptoms. A person may be quarantined for the incubation period and a few additional days to be cautious. However, the length of the incubation period for an infectious agent may be unclear, and the person might not know when they were exposed.

Question 38

Is it worthwhile to quarantine a patient?

Answer 38

It depends on the infection. With many infections, by the time a person exhibits clinical illness, they have already exposed many people and quarantine will not be as effective. However, quarantine can be an important infection prevention tool. The 2012 MERS-CoV epidemic was contained through strong infection control measures that reduced contacts of travelers who may have been exposed

Question 39

Latency period

Answer 39

The incubation period of noninfectious diseases. When a person is exposed to a carcinogen or toxin, the disease will usually manifest months or years later. Mesothelioma from asbestos exposure may occur 20-30 years after exposure

Question 40

Epidemic curve

Answer 40

The distribution of the times of onset of the disease

Question 41

Single exposure, common vehicle epidemic epidemic curve

Answer 41

The epidemic curve represents the distribution of the incubation periods. The interval from the point of onset for each case is the incubation period of each person, since each person received the same exposure. The number of cases will increase explosively after a certain time interval

Question 42

3 critical variables in investigating an outbreak or epidemic

Answer 42

1. When did the exposure take place?
2. When did the disease begin?
3. What was the incubation period for the disease?
   If we know any two of these variables, we can calculate the third

Question 43

How is attack rate calculated?

Answer 43

Number of people at risk in whom a certain illness develops divided by total number of people at risk

Question 44

Attack rate

Answer 44

Used to compare the risk of disease in groups with different exposures. The attack rate can be specific for a given exposure, like the food-specific attack rate. The time period is implicit in the attack rate since the exposure is common and the illness is acute

Question 45

Food specific attack rate

Answer 45

The attack rate in people who ate a certain food. It’s calculated by number of people who ate a certain food and became ill divided by total number of people who ate that food

Question 46

Primary case

Answer 46

A person who acquires the disease from that exposure (for example, from contaminated food)

Question 47

Secondary case

Answer 47

A person who acquires the disease from exposure to a primary case

Question 48

Secondary attack rate

Answer 48

Defined as the attack rate in susceptible people who were not exposed to the suspected agent who have been exposed to a primary case. It’s a good measure of person to person spread of disease after the disease has been introduced into a population. The secondary attack rate can also be used in noninfectious diseases to determine the extent to which a disease clusters among first degree relatives of an index case (heritability). This can provide clues regarding the genetic or environmental factors that contribute to a disease

Question 49

When a disease appears to have occurred at more than an endemic rate and we wish to investigate disease occurrence, what 3 questions do we ask?

Answer 49

1. Who was attacked by the disease?
2. When did the disease occur?
3. Where did the cases arise?

Question 50

Which host characteristics are related to disease risk?

Answer 50

Sex, age, race, behavioral risk factors (like smoking). For example, the highest rate of pertussis is in infants less than 6 months of age. Gonorrhea is more common in men than in women, but women are more likely to be asymptomatic and therefore less likely to be diagnosed

Question 51

Why does when a disease occurred matter?

Answer 51

Certain diseases occur with a certain periodicity. There is often a seasonal pattern to the temporal variation, for example, respiratory disease is most common during the winter months. There are also different trends in disease incidence over time. For example, deaths from AIDS increased for years, then began to decrease in 1996, due to new therapy and health education efforts

Question 52

Why does the location of disease matter?

Answer 52

Disease is not randomly distributed in time or place. Lyme disease tends to cluster around the northeast coast of the US and in the north-central part of the country, with another cluster along the pacific coast

Question 53

Cross tabulation

Answer 53

When they are multiple possible causal agents, cross tabulation is helpful in determining the cause of the outbreak. In August 1974, a streptococcal pharyngitis outbreak occurred in a Florida jail. The causes were narrowed down to an egg salad sandwich and a beverage served for lunch on a specific day, and cross tabulation was used to determine the cause. The attack rates in those who ate egg salad were compared with those who did not, but researchers also looked separately at those who drank the beverage and those who didn’t. Through the data, it was determined that drinking the beverage did not increase the risk of illness, but eating the egg salad did. The egg salad was implicated as the source of the infections

Question 54

What are host factors that can increase susceptibility or severity of disease? (5)

Answer 54

1. Age- immune system declines with age, and newborns have a developing immune system
2. Genetic factors
3. Lifestyle factors like smoking (host behaviors), impact both severity of disease and exposure to disease
4. Pre-existing medical conditions
5. Immunity- prior disease/vaccination

Question 55

What are agent factors that may promote/reduce spread of disease? (5)

Answer 55

1. The ability of the agent to change and evade the immune system
2. Severity of the disease- a severe disease is more easily maintained
3. Mode of transmission- airborne transmission is more common than sexual transmission
4. How long the pathogen remains in the body- infectious period
5. Whether the agent causes a lasting immune response

Question 56

R0

Answer 56

Measures the potential for transmission of a communicable disease in a population where everyone is susceptible to the infection. If R0 is 5, an infected person will go on to infect 5 other people in an immunologically naive population. The R0 value is affected by mode/route of transmission and period of communicability

Question 57

Mode/route of transmission

Answer 57

They way a disease is spread- direct or indirect spread

Question 58

Period of communicability

Answer 58

Some diseases are infectious before they become symptomatic. Measles has an infectious period that begins 4 days before the person develops symptoms

Question 59

Which environmental factors that may promote/reduce the spread of disease? (3)

Answer 59

1. Environmental factors
2. Climate- temperature and humidity can impact how long pathogens can survive on a surface
3. Ecosystem- zoonotic factors, social factors, quality of healthcare/testing

Question 60

Elimination

Answer 60

Disease is no longer in a geographic region. If the disease circulates for longer than a year, you lose elimination status

Question 61

Eradication

Answer 61

The disease no longer in the world