HUMAN BIOLOGY

ImmunIty and dIsease 173

endemic disease Disease

that occurs more or less

continously.

epidemic A disease rate

that exceeds what would be

expected.

nosocomial infection An

infection acquired in a

hospital.

pandemic Widespread

occurrence of an epidemic

disease.

sporadic disease A

disease that breaks out

irregularly.

virulence A measure of the

likelihood that a pathogen

will make its host seriously ill.

Infectious diseases occur in different patterns. In an

epidemic, a disease rate increases to a level above what

experience would predict. the 2011 cholera epidemic in haiti

is an example. When an epidemic breaks out in different

places around the world in a given time span, the outbreaks

collectively are said to be pandemic.

hIV/aIds is pandemic, as is the

viral h 1 n1 “swine flu” that spread

around the world in 2009. a sporadic

disease, such as whooping cough,

breaks out irregularly and affects

relatively few people. an endemic

disease, such as the common cold,

occurs more or less all the time. many

diseases listed in the chart in Figure

9.22 are endemic in various parts of

the world.

disease pathogens are ranked

according to their virulence—how

likely it is that the host will fall

seriously ill. Virulence depends

on how fast the pathogen can

invade tissues, how much damage

it causes, and which tissues it

targets. For example, a virus that can cause pneu monia is

more virulent than one that causes the sniffles. rabies is

extremely virulent because it targets the brain.

a Constant Enemy: infectious Disease

9.11

Type of Estimated Deaths

Disease Pathogen per Year

Diarrheas Protozoa, virus, 31 million

(includes amoebic and bacteria

dysentery,

cryptosporidiosis)

Various respiratory Virus, bacteria 71 million

infections

(pneumonia, viral

influenza, diphtheria,

strep infections)

Malaria Protozoa 2.7 million

Tuberculosis Bacteria 2.4 million

Hepatitis Virus 1–2 million

(includes A, B,

C, D, E)

Measles Virus 220,000

Schistosomiasis Worm 200,000

Whooping cough Bacteria 100,000

Hookworm Worm 50,000 1

Figure 9.22 Worldwide, diarrheal diseases and respiratory

infections take more than 100 million lives each year. The

statistics shown here, from the 2010 World Health Report, do

not include cases of HIV/AIDS.

Figure 9.21 Many infections

spread by contact or when a

pathogen is inhaled. A Athlete’s

foot. B Droplets spewed by a

sneeze. C Clostridium difficile

bacteria (colored green here)

cultured from the colon of an

infected hospital patient.

A B

C

A: Dr. P. Marazzi/Science Source; B: Yoav Levy/Phototake; C: Paul Gunning/

Science Source

FOCUS ON HUmAN impACT

your whole life long, infectious diseases will be challenging
your immune system. We define “infectious disease” as
a disease that can be transmitted from person to person.
there are four common modes of transmission.

1. Direct contact with a pathogen, as by touching
   open sores or body fluids from an infected person.
   (“Contagious” comes from the Latin contagio, meaning
   touch or contact.)


2. indirect contact, as by touching doorknobs, tissues,
   diapers, or other objects previously in contact with an
   infected person. athlete’s foot is an example (Figure 9.21a).


3. inhaling pathogens, such as cold and flu viruses, that
   have been spewed into the air by uncovered coughs or
   sneezes (Figure 9.21B).


4. Contact with a vector, such as a fly or mosquito,
   which carries a pathogen from an infected person or
   contaminated material to new hosts. In some cases, part
   of the pathogen’s life cycle occurs inside the vector, which
   is an intermediate host. For example, mosquitoes are the
   intermediate hosts for the West nile virus.
   a nosocomial infection is one acquired in a hospital.
   two of the most serious ones are mrsa infections and
   colon inflammation caused by Clostridium difficile bacteria
   (Figure 9.21C). these and other nosocomial infections are
   so common because anyone sick enough to be hospitalized
   may have a compromised immune system, and invasive
   medical procedures give bacteria easy access to tissues.
   also, as you might guess, intensive use of antibiotics in
   hospitals increases the chances that antibiotic-resistant
   pathogens will be present there.

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