1. 32.1 The Science of Epidemiology
The study of the occurrence, distribution, and determinants of health and disease in a population
In developed countries infectious diseases cause fewer deaths than noninfectious diseases
Worldwide, infectious disease accounts for 30% of all deaths
Even in developed countries, new diseases are emerging
Example: West Nile fever
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2. 1000 80 60 800 Mortality per 100,000 40 600 20 Mortality per 100,000
Figure 32.1
1000 80 60
800
Mortality per 100,000 40
600 20
Mortality per 100,000
1970
1980
1990
400
Figure 32.1 Deaths due to infectious disease in the United States.
200
1900
1920
1940
1960
1980
Year
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3. 32.2 The Vocabulary of Epidemiology
A disease is an epidemic when it occurs in a large number of people in a population at the same time (Figure 32.2)
A pandemic is widespread, usually worldwide
An endemic disease is constantly present in a population, usually at low incidences
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4. Endemic disease Epidemic disease Pandemic disease Figure 32.2
Figure 32.2 Endemic, epidemic, and pandemic disease.
Endemic disease
Epidemic disease
Pandemic disease
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5. 32.2 The Vocabulary of Epidemiology
A disease outbreak occurs when a number of cases of a disease are reported in a short period of time
Mortality is the incidence of death in a population
Morbidity of a disease refers to the incidence of disease including fatal and nonfatal diseases
Reservoirs are sites in which infectious agents remain viable and from which infection of individuals can occur
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6. 32.3 Disease Reservoirs and Epidemics
Zoonosis is any disease that primarily infects animals, but is occasionally transmitted to humans
Control of a zoonotic disease in the human population may not eliminate the disease as a potential public health problem
Certain infectious diseases have complex life cycles involving an obligate transfer from a nonhuman host to humans followed by transfer back to the nonhuman host
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7. 32.4 Infectious Disease Transmission
Epidemiologists follow transmission of a disease by correlating geographic, climatic, social, and demographic data (Figure 32.3)
Used to identify possible modes of transmission
For example, a disease limited to a tropical area may suggest something about its vector (e.g., malaria)
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8. Figure 32.3 50 45 40 35 30
Cases 25 20 15 10
Figure 32.3 The incidence of California encephalitis in the United States by month and year. 5
2000
2001
2002
2003
Month–year
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9. 32.4 Infectious Disease Transmission
Pathogens can be classified by their mechanism of transmission, but all mechanisms have the following stages in common:
Escape from host
Travel
Entry into new host
Pathogen transmission can be direct or indirect
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10. 32.4 Infectious Disease Transmission
Direct host-to-host transmission
Infected individual transmits a disease directly to a susceptible host without the assistance of an intermediary (e.g., flu, common cold, STDs, ringworm)
Indirect host-to-host transmission
Occurs when transmission is facilitated by a living or nonliving agent
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11. Vector borne disease
Pathogens are carried by someone or something that does not contract the disease
Arthropods (mosquitos, mites, ticks, fleas)
Water
Medical equipment
Vector borne diseases tend to be more severe since there is no need to keep previous host healthy enough to transmit pathogen

12. Transmission and symptoms
Successful pathogens tend to manipulate the host immune system to increase the likelihood of transmission to new hosts
respiratory pathogens cause coughing or sneezing
water-borne pathogens cause diarrhea
arthropod-borne pathogens often cause lethargy which allows bites by other insects
STDs cause lesions on genitals which secrete pathogen during sex with new potential host

13. 32.5 The Host Community
Coevolution of a host and its parasite is common
Virulence of the parasite in host-to-host transmission diminishes and resistance of the host increases (e.g., myxoma virus introduced to control rabbits in Australia; Figure 32.5)
A host-to-host pathogen that kills its host before it can infect another host may become extinct
If a pathogen does not rely on host-to-host transmission it may remain extremely virulent (e.g., E. coli in hospitals)
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14. 100 100 Rabbit mortality 80 60 90 Rabbit mortality (%)
Figure 32.5
100
100
Rabbit mortality 80 60 90
Rabbit mortality (%)
Virus virulence (%) 40
Virus virulence
Figure 32.5 Myxoma virus, virulence, and Australian rabbit susceptibility. 20 80 1 2 3 4 5 6
Years
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15. 32.5 The Host Community
Herd immunity is the resistance of a group to infection due to immunity of a high proportion of the group (Figure 32.6)
If a high proportion of individuals are immune to an infection then the whole population will be protected
Immunized people protect nonimmunized people because the pathogen cannot be passed on and the cycle of infectivity is broken
Diseases such as influenza tend to occur in cycles
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16. Susceptible B A Infected Immune C Figure 32.6
Figure 32.6 Herd immunity and transmission of infection. C
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17. 32.6 The HIV/AIDS Pandemic
Acquired immunodeficiency syndrome (AIDS) is a viral disease that attacks the immune system
First reported cases were in the U.S. in 1981
At least 70,000,000 people have been infected worldwide with HIV
More than 25,000,000 people have died from AIDS
Studies in the U.S. suggested the virus was transmitted through sexual contact or blood (Figure 32.8)
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18. Total AIDS cases, United States
Figure 32.7
80,000
70,000
60,000
50,000
Total AIDS cases, United States
40,000
30,000
Figure 32.7 Annual newly diagnosed cases of acquired immunodeficiency syndrome (AIDS) since 1981 in the United States.
20,000
10,000
1981
1985
1990
1995
2000
2005
Year
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19. 71.3% 82.7% Males Females 4% 0.89% 0.3% Male-to-male sexual contact
Figure 32.8 4%
0.3%
0.89%
Male-to-male sexual contact
9.9%
16.5%
Heterosexual contact
14.5%
Injection drug use
71.3%
82.7%
Male-to-male sexual contact and injection drug use
Figure 32.8 Distribution of AIDS cases by risk group and sex in adolescents and adults in the United States, 2007.
Males
Females
Other
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20. 32.7 Healthcare-Associated Infections
Healthcare-associated infections (HAIs)
Result from an infectious agent acquired at a healthcare facility (Figure 32.9)
Also referred to as nosocomial infections
There are about 1,700,000 HAIs per year in the U.S.
Some are acquired from other patients, but others are caused by pathogens that are selected for and maintained by the hospital environment
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21. Urinary tract infections 32%
Figure 32.9
Blood- stream infections 14%
Other 17%
Surgical site infections 22%
Urinary tract infections 32%
Figure 32.9 Healthcare-associated infections.
Respiratory tract infections 15%
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22. 32.7 Healthcare-Associated Infections
Infectious diseases are spread in hospitals for several reasons:
Patients have low resistance to infectious disease
Healthcare facilities treat infectious disease patients
Multiple patients in the same room
Healthcare personnel move from patient to patient
Healthcare procedures may breach the skin and introduce infection
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23. 32.7 Healthcare-Associated Infections
Infectious diseases are spread in hospitals for several reasons (cont’d):
Newborn infants are susceptible to infection
Surgical procedures expose organs to contamination
Certain drugs increase a patient’s susceptibility to infection
Use of antibiotics has selected for antibiotic-resistant organisms
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24. 32.9 Global Health Considerations
Infectious diseases in Americas vs. Africa (Figure 32.10)
Death rate in Africa is much higher
Most African deaths are due to infectious diseases (10 as many as in the Americas)
Concern for people traveling to other areas
Travelers can be immunized
Drink only decontaminated water
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25. Africa 2004: 11.2 million deaths
Figure 32.10
Africa 2004: 11.2 million deaths
Injuries 5%
Intentional 3%
Other 8%
Respiratory 3%
Cardiovascular diseases 11%
Infections 56%
Maternal and perinatal 10%
Cancer 4%
The Americas 2004: 6.2 million deaths
Intentional 4%
Other 13%
Infections 10%
Figure Causes of death in Africa and the Americas, 2004.
Injuries 10%
Cancer 20%
Diabetes 4%
Cardiovascular diseases 32%
Maternal and perinatal 3%
Respiratory 6%
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