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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

Endemic disease Epidemic disease Pandemic disease Figure 32.2 Figure 32.2 Endemic, epidemic, and pandemic disease. Endemic disease Epidemic disease Pandemic disease © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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) © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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

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

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) © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

Susceptible B A Infected Immune C Figure 32.6 Figure 32.6 Herd immunity and transmission of infection. C © 2012 Pearson Education, Inc.

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) © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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% © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 32.10 Causes of death in Africa and the Americas, 2004. Injuries 10% Cancer 20% Diabetes 4% Cardiovascular diseases 32% Maternal and perinatal 3% Respiratory 6% © 2012 Pearson Education, Inc.