TRANSMISSION AND NATURAL HISTORY OF DISEASE

Transmission of infectious diseases in humans requires the following:

1. An agent capable of infecting humans;

2. A source: an infected host, a reservoir of infection;

3. A portal of exit from the source;

4. A suitable means of transmission;

5. A portal of entry into a new host;

6. A susceptible host.

The study of epidemiology is often approached through the study of three important factors: agent, host, and environment. Each will be discussed here with particular reference to infectious diseases. The host will be discussed in section 6 and the environment in section 2. The agent is discussed below.

1. An Agent Capable of Infecting Humans

Agent refers to that which is necessary to bring about a particular disease, disability, or pathological state in a susceptible host. The agent may be biological, physical, chemical, or nutritional in nature. Note: an agent may be defined by its presence, absence, excess, or deficiency.

Questions -

Question 1: Give examples of diseases caused by each of the types of agents mentioned above (biological, physical. chemical, nutritional).

Note that an agent is most easily defined in terms of infectious and acute diseases. Agents are more difficult to determine in non-infectious and chronic diseases.

Question 2: A. Discuss measles in terms of infectivity, pathogenicity, and virulence.

B. As a public health expert, what effort (priority) would you put into controlling an agent with very high infectivity but low pathogenicity? Would you modify your approach if this agent was virulent although of low pathogenicity? Can you cite an example of such a disease?

C. Are the agents that cause the common cold highly infective? Highly pathogenic? Highly virulent? Why or why not?

Question 3: What implications does the period of infectiousness have for the control of transmission of these diseases? (That is, when would isolation not be warranted?) How are chronic earners of typhoid and hepatitis B managed?

Question 4: Controlling a disease for which humans are the reservoir is considered easier than controlling a disease for which other animals are the reservoir. What are the factors necessary before a disease in which humans are the reservoir can be eradicated? How would agent characteristics such as infectiousness, pathogenicity, and virulence affect the probability of achieving eradication? Is it possible to eradicate a disease for which humans are not the reservoir? Has it been achieved at a local/national/international level?

Question 5:

a) Suggest a disease affecting humans in which the portal of exit is:

- the respiratory passages

- the alimentary canal

- skin lesions

b) How can diseases using these portals of exit be controlled? For which disease(s) have these control measures been most successful?

Question 6: For each of the following patterns of transmission, identify an agent and how control of the disease caused by it has been or could be controlled.

a. Human to human, directly;

b. Animal to human, directly;

c. Human to human, by means of a vehicle;

d. Human to human, by means of a vector.

Question 7: List the possible portals of entry for influenza, gonorrhea, syphilis, hepatitis, streptococcus, and plague. Include both usual and alternative portals of entry.

Question 8: How are unapparent infections identified?

Question 9: Poor socioeconomic environment is implicated in the relatively high incidence, severe course, and/or poor outcome of certain diseases (e.g., tuberculosis). Can you think of any infectious disease facilitated by a superior social or economic environment? Discuss.

Question 10: What aspects, of slow and latent agents make them particularly difficult to study using an epidemiologic strategy? Can you suggest approaches which might be used for identifying slow or latent agents responsible for disease?

Question 11: Measles is an example of a disease having modified epidemiology due to human intervention. Given the information above, how can measles incidence be further reduced in the U.S.?

Question 12: In your own words, describe the natural history of hepatitis C virus infection, with reference to each of the stages in the natural history timeline. (You likely will need to reference an external source to complete this question -- Google is your friend!)

Question 13: For hepatitis C virus infection, describe the primary, secondary, and tertiary preventions.

Question 14: Based on the information in the table below, what is the probability of surviving for 3 years?

Question 15: In your own words, explain how differences in the detection and diagnosis of disease can influence measures of prognosis.

Word limit 100 per question.

Attachment:- Assignment File.rar