(Data from Centers for Disease Control and Prevention: Summary of notifiable diseases, United States, 1992. MMWR 41:41, 1992; and Summary of notifiable diseases, United States, 1997. MMWR 46:18, 1998.)

Figure 3-2 shows the decline in the reported incidence and mortality from diphtheria in the United States. The data in this figure are presented in the form of a semilogarithmic graph, with a logarithmic scale used for the vertical y-axis and an arithmetic scale for the horizontal x-axis. The figure illustrates one advantage of using a logarithmic scale: The lines showing incidence and mortality trace an approximately parallel decline. On a logarithmic scale, this means that the decline in rates was proportional, so that the percentage of cases that resulted in death—the case fatality ratio—remained relatively constant at about 10% over the years shown. This relative constancy suggests that prevention of disease, rather than treatment of people who were ill, was responsible for the overall reduction in diphtheria mortality in the United States.

Figure 3-2 Incidence rates, mortality rates, and case fatality ratios for diphtheria in the United States, by year of report, 1920-1975.

(Data from Centers for Disease Control and Prevention: Diphtheria surveillance summary. Pub No (CDC) 78-8087, Atlanta, 1978, CDC.)

Seasonal Variation

Many infectious diseases show a strong seasonal variation, with periods of highest incidence usually depending on the route of spread. To determine the usual number of cases or rates of disease, epidemiologists must therefore incorporate any expected seasonal variation into their calculations.

Infectious diseases that are spread by the respiratory route, such as influenza, colds, measles, and varicella (chickenpox), have a much higher incidence in the winter and early spring in the Northern Hemisphere. Figure 3-3 shows the seasonal variation for varicella in the United States, by month, over a 6-year period. Notice the peaks after January and before summer of each year. Such a pattern is thought to occur during these months because people spend most of their time close together indoors, where the air changes slowly. The drying of mucous membranes, which occurs in winter because of low humidity and indoor heating, may also play a role in promoting respiratory infections. Since the introduction of varicella vaccine, this seasonal pattern has been largely eliminated.

Figure 3-3 Incidence rates of varicella (chickenpox) in the United States, by month of report, 1986-1992.

(Data from Centers for Disease Control and Prevention: Summary of notifiable diseases, United States, 1992. MMWR 41:53, 1992.)

Diseases that are spread by insect or arthropod vectors (e.g., viral encephalitis from mosquitoes) have a strong predilection for the summer or early autumn. Lyme disease, spread by Ixodes ticks, is usually acquired in the late spring or summer, a pattern explained by the seasonally related life cycle of the ticks and the outdoor activity of people wearing less protective clothing during warmer months.

Infectious diseases that are spread by the fecal-oral route are most common in the summer, partly because of the ability of the organisms to multiply more rapidly in food and water during warm weather. Figure 3-4 shows the summer seasonal pattern of waterborne outbreaks of gastrointestinal disease. The peak frequency of outbreaks attributable to drinking water occurs from May to August, whereas the peak for outbreaks attributable to recreational water (e.g., lakes, rivers, swimming pools) occurs from June to October.

Figure 3-4 Incidence of waterborne outbreaks of gastrointestinal disease in the United States, by month of report, 1991-1992.

(Data from Centers for Disease Control and Prevention: Surveillance for waterborne disease outbreaks, United States, 1991-1992. MMWR 42(SS-5):1, 1993.)

Figure 3-5 shows a late-summer peak for aseptic meningitis, which is usually caused by viral infection spread by the fecal-oral route or by insects. Figure 3-6 shows a pattern that is similar but has sharper and narrower peaks in late summer and early autumn. It describes a known arthropod-borne viral infection caused by California-serogroup viruses of the central nervous system.

Figure 3-5 Incidence rates of aseptic meningitis in the United States, by month of report, 1986-1992.

(Data from Centers for Disease Control and Prevention: Summary of notifiable diseases, United States, 1992. MMWR 41:20, 1992.)

Figure 3-6 Incidence of central nervous system infections caused by California-serogroup viruses in the United States, by month of report, 1981-1997.

(Data from Centers for Disease Control and Prevention: Summary of notifiable diseases, United States, 1992. MMWR 41:18, 1992; and Summary of notifiable diseases, United States, 1997. MMWR 46:20, 1998.)

Because the peaks of different disease patterns occur at different times, the CDC sometimes illustrates the incidence of diseases by using an “epidemiologic year.” In contrast to the calendar year, which runs from January 1 of one year to December 31 of the same year, the epidemiologic year for a given disease runs from the month of lowest incidence in one year to the same month in the next year. The advantage of using the epidemiologic year when plotting the incidence of a disease is that it puts the high-incidence months near the center of a graph and avoids having the high-incidence peak split between the two ends of the graph, as would occur with many respiratory diseases if they were graphed for a calendar year.

Other Types of Variation

Health problems can vary by the day of the week; Figure 3-7 shows that recreational drowning occurs more frequently on weekends than on weekdays, presumably because more people engage in water recreation on weekends.

Figure 3-7 Number of drownings at recreation facilities of U.S. Army Corps of Engineers, by day of week of report, 1986-1990.

(Data from Centers for Disease Control and Prevention: Drownings at U.S. Army Corps of Engineers recreation facilities, 1986-1990. MMWR 41:331, 1992.)

3 Identification and Documentation of Outbreaks

An epidemic, or disease outbreak, is the occurrence of disease at an unusual (or unexpected) frequency. Because the word “epidemic” tends to create fear in a population, that term usually is reserved for a problem of wider-than-local implications, and the term “outbreak” typically is used for a localized epidemic. Nevertheless, the two terms often are used interchangeably.

It is possible to determine that the level of a disease is unusual only if the usual rates of the disease are known and reliable surveillance shows that current rates are considerably elevated. To determine when and where influenza and pneumonia outbreaks occur, the CDC uses a seasonally adjusted expected percentage of influenza and pneumonia deaths in the United States and a number called the epidemic threshold to compare with the reported percentage. (Pneumonias are included because influenza-induced pneumonias may be signed out on the death certificate as “pneumonia,” with no mention of influenza.)

Figure 3-8 provides data concerning the expected percentage of deaths caused by pneumonia and influenza in 122 U.S. cities for 1994 through 2000. The lower (solid) sine wave is the seasonal baseline, which is the expected percentage of pneumonia and influenza deaths per week in these cities. The upper (dashed) sine wave is the epidemic threshold, with essentially no influenza outbreak in winter 1994-1995, a moderate influenza outbreak in winter 1995-1996, and major outbreaks in the winters of 1996-1997, 1997-1998, and 1998-1999, as well as in autumn 1999. No other disease has such a sophisticated prediction model, but the basic principles apply to any determination of the occurrence of an outbreak.

Figure 3-8 Epidemic threshold, seasonal baseline, and actual proportion of deaths caused by pneumonia and influenza in 122 U.S. cities, 1994-2000.
The epidemic threshold is 1.645 standard deviations above the seasonal baseline. The expected seasonal baseline is projected using a robust regression procedure in which a periodic regression model is applied to observed percentages of deaths from pneumonia and influenza since 1983.

(Data from Centers for Disease Control and Prevention: Update: influenza activity—United States and worldwide, 1999-2000. MMWR 49:174, 2000.)

Surveillance for Bioterrorism

For at least a century, epidemiologists have worried about the use of biologic agents for military or terrorist purposes. The basic principles of disease surveillance are still valid in these domains, but there are special concerns worth mentioning. The most important need is for rapid detection of a problem. With regard to bioterrorism, special surveillance techniques are being developed to enable rapid detection of major increases in the most likely biologic agents ³ (Box 3-1). Detection is made more difficult if the disease is scattered over a wide geographic area, as with the anthrax outbreak in the United States after terrorist attacks in late 2001.

Box 3-1 Diseases Considered Major Threats for Bioterrorism

A technique developed for more rapid detection of epidemics and possible bioterrorism is syndromic surveillance.³ The goal of this surveillance is to characterize “syndromes” that would be consistent with agents of particular concern and to prime the system to report any such syndromes quickly. Rather than trying to establish a specific diagnosis before sounding an alert, this approach might provide an early warning of a bioterrorism problem.