| Course | NU 602 Epidemiology, Research, and Theory |
|---|---|
| Unit | Unit 2 |
| Paper type | Prevalence and duration brief |
| Length | About 820 words, 5 pages |
| Format | APA 7 student paper |
| School | Herzing University |
| Program | MSN |
| Updated | October 2026 |
Free sample paper for NU 602 Unit 2
Why 300 New Cases a Year Can Mean 23 People or 9,000: Prevalence and Duration for Treated Lyme Disease and HIV Infection
Student Name
Department of Nursing, Herzing University
NU 602: Epidemiology, Research, and Theory
Instructor Name
January 28, 2026
Why 300 New Cases a Year Can Mean 23 People or 9,000: Prevalence and Duration for Treated Lyme Disease and HIV Infection
The Puzzle
Consider an illustrative metropolitan region of 1,000,000 people in which two conditions each produce about 300 new diagnoses a year: Lyme disease and HIV infection. The numbers are chosen to be similar for the purpose of the comparison; both are within the range seen in some U.S. regions. A health planner looking only at new cases would conclude that the two conditions place similar demands on the region's health services. They do not. On any given day, the region has perhaps two dozen people being treated for Lyme disease and several thousand people living with HIV. The difference has nothing to do with how often people acquire the conditions. It is explained by how long each condition lasts once a person has it.
The Relationship
When a condition is uncommon and its incidence and duration have been roughly stable for a long time, prevalence is approximately the product of incidence and average duration: P is about I times D. The more exact form, which holds when prevalence is not small, relates the prevalence odds, P divided by (1 minus P), to incidence times duration (Freeman & Hutchison, 1980). The relationship is stated once here and then used; the point is not the formula but what it says about time. Every new case enters the pool of prevalent cases and stays until it leaves, by recovery, death, or moving away. A condition that people leave quickly keeps a small pool; one that people live with for decades fills a large one (Celentano & Szklo, 2019).
Lyme Disease: A Short Stay in the Pool
Early Lyme disease, diagnosed and treated with a course of antibiotics, usually resolves within a few weeks. Taking an average duration of about four weeks, or 0.077 years, the approximate prevalence is 300 new cases a year times 0.077 years, which is about 23 people with active disease at any time, or about 2.3 per 100,000. Most of those 23 are different people from month to month. The condition generates clinic visits and prescriptions spread over the season but almost no ongoing care, and its prevalence tells a planner very little about its importance.
HIV Infection: A Long Stay in the Pool
With effective antiretroviral treatment, HIV infection is a chronic condition, and people diagnosed today can expect to live for decades. Estimates from a large North American cohort found that the expected remaining lifespan of a 20-year-old on treatment rose from about 36 years in the early 2000s to about 51 years by 2006 to 2007 (Samji et al., 2013). Taking an illustrative average duration of 30 years after diagnosis, and assuming that 300 new diagnoses a year had held steady for that long, prevalence would be about 300 times 30, or 9,000 people, about 900 per 100,000. Each of them needs regular clinical visits, laboratory monitoring, and daily medication.
Where the Steady-State Assumption Breaks
The calculation for HIV depends on an assumption that rarely holds: that incidence and duration have been stable over the whole period. In most U.S. regions, new HIV diagnoses were higher in earlier decades, and survival was much shorter before effective treatment. Actual prevalence today therefore reflects past incidence and past survival, not the current figures. The formula is best used as a way of reasoning about direction, not as a way to estimate a precise count. For Lyme disease, whose duration is short, the assumption matters much less, because the pool turns over within weeks.
Treatment Success Raises Prevalence
The relationship produces a result that surprises many clinicians. Suppose better treatment doubles average survival after diagnosis from 15 to 30 years while incidence stays at 300 a year. Prevalence, at steady state, doubles from 4,500 to 9,000. Now suppose that prevention also succeeds and incidence falls by a third, to 200 a year, while duration stays at 30 years. Prevalence would be 6,000, still higher than the 4,500 of the earlier period. A rising count of people living with HIV can therefore be a sign of success, reflecting longer lives, even as fewer people are newly infected. A planner who reads rising prevalence as a failure of prevention has confused the two measures.
What This Means for Planning
Incidence is the right measure for judging prevention and for estimating how many people will need a diagnosis and a first course of treatment. Prevalence is the right measure for estimating ongoing care: clinic capacity, case management, and medication budgets. For Lyme disease, the two measures lead to similar planning, because every case is brief. For HIV, they lead to very different planning, because each new case adds a person who will need care for decades. The same 300 new cases a year call for a seasonal clinic response in one case and a permanent program in the other.
References
Celentano, D. D., & Szklo, M. (2019). Gordis epidemiology (6th ed.). Elsevier.
Freeman, J., & Hutchison, G. B. (1980). Prevalence, incidence and duration. American Journal of Epidemiology, 112(5), 707-723. https://doi.org/10.1093/oxfordjournals.aje.a113043
Samji, H., Cescon, A., Hogg, R. S., Modur, S. P., Althoff, K. N., Buchacz, K., Burchell, A. N., Cohen, M., Gebo, K. A., Gill, M. J., Justice, A., Kirk, G., Klein, M. B., Korthuis, P. T., Martin, J., Napravnik, S., Rourke, S. B., Sterling, T. R., Silverberg, M. J., ... North American AIDS Cohort Collaboration on Research and Design of IeDEA. (2013). Closing the gap: Increases in life expectancy among treated HIV-positive individuals in the United States and Canada. PLoS ONE, 8(12), Article e81355. https://doi.org/10.1371/journal.pone.0081355
The Herzing NU 602 Unit 2 instructions
In Unit 2 you explain the relationship between incidence, prevalence and duration, usually by comparing two conditions. Expect to state the relationship and its assumptions, apply it with numbers, discuss when it does not hold and explain the implications for health planning or practice. Many versions ask you to choose conditions with very different durations so the effect is clear. Check whether your section wants the formula written out or explained in prose; most accept both if the logic is visible. Use realistic figures from surveillance data or the literature, or label numbers as illustrative. The steady-state assumption is often the point graders test, so say when it applies and when changing incidence or survival makes the simple relationship misleading.
How this NU 602 Unit 2 example is built
The brief opens with a puzzle, two conditions with the same yearly diagnoses and very different numbers of people affected, which gives the reader a reason to care about the relationship. The relationship section states the formula and its conditions in two sentences. Two parallel sections work each condition, with duration stated and prevalence calculated. The section on where the assumption breaks explains that HIV incidence and survival have both changed, so the simple product overstates or understates the pool depending on the period. The treatment section works a short example showing that doubling survival doubles prevalence even if incidence does not change. The planning section explains that incidence guides prevention and first treatment, while prevalence guides ongoing care, clinic capacity and drug budgets.
Where the points sit in the NU 602 Unit 2 rubric
Herzing tends to grade this brief on the explanation of concepts, the correct application of the relationship, attention to assumptions, implications for practice and writing. Concept points require accurate definitions of incidence, prevalence and duration. Application earns full marks when calculations are correct and the units are clear. The assumptions criterion often separates strong papers, rewarding a clear statement of when steady state holds and what happens when it does not. Implication points go to concrete planning consequences. Writing covers clarity for a reader who is not an epidemiologist and APA 7 references, including Freeman and Hutchison's 1980 paper and the Samji cohort on life expectancy with HIV.
NU 602 Unit 2 help before you submit
Briefs in Unit 2 commonly lose points by stating the formula without explaining its assumptions. Another frequent issue is choosing two conditions with similar durations, which hides the effect the assignment is built to show. Students often treat the relationship as exact even when incidence has changed sharply. Calculations without units are hard to grade. The idea that better treatment raises prevalence is sometimes missed or stated without an example. Planning implications are often too general. Using outdated survival figures for chronic conditions skews the numbers. Finally, label illustrative figures clearly, because graders need to know whether a number comes from surveillance data or was chosen for the example.
Get a NU 602 Unit 2 example written to your instructions
Send the Unit 2 instructions and the rubric from your NU 602 classroom, plus the conditions your section names or the population you want compared. We write a custom example that holds new cases fixed, varies duration alone, reads a climbing figure correctly and converts the result into service demand, returned in 24 to 48 hours. The first custom sample is free.
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NU 602 Unit 2 questions, answered
Which two conditions work best for this comparison?
Pick a pair that genuinely differs in how long people remain cases. A common infection that resolves within days set against a chronic condition managed for thirty years shows the effect without any strain. Avoid pairing two chronic conditions, since the contrast flattens and the brief ends up arguing about small differences rather than about the principle your instructor is looking for.
Do I need the formula, or is prose enough?
Include the relationship and then spend your space on what it means. A brief that reproduces the expression and stops has answered a mathematical question nobody asked, while a brief that never states it looks as though the writer was avoiding the mechanism. State it once, work two short comparisons through it, and let the interpretation carry the remainder of the paper.
Why does this matter for practice?
Because planning follows duration rather than incidence. A condition producing few new cases each year, with people living decades after diagnosis, generates enormous demand for appointments, medication and support, while a condition with far more new cases and rapid resolution generates comparatively little. Budgeting and staffing off new-case counts alone is how services end up structurally short.
Where can I find a free NU 602 Unit 2 sample paper?
Read the complete NU 602 Unit 2 Lyme and HIV brief here at no charge, the steady-state relationship, the worked numbers and the planning section. A custom brief can compare two other conditions your section suggests.
Why does better treatment increase disease prevalence?
Prevalence depends on how long people live with a condition. If treatment extends survival but does not cure, more people remain in the pool each year. This sample shows that doubling survival with HIV roughly doubles prevalence even when new diagnoses stay the same.