Awakening with sniffles and a runny nose...find-

ing a moldy surprise in the refrigerator behind the milk
carton... wondering if that milk has gone sour. No
matter that we cannot see them, the microbes, includ-
ing viruses, fungi, and bacteria, are always with us.
Microbiology is the study of microorganisms (also
called microbes) and of their place or role in their
environment. Sometimes, the environment of mi-
crobes is us, people, and that will be our focus.
Although there are many perspectives in the field of
microbiology, we will concentrate on the microorgan-
isms that affect the functioning of the human body.
These microbes include our normal flora and those
that are capable of causing human disease. A disease is
any disruption of normal body functioning. Many dis-
eases are not caused by microorganisms, but those that
are may be called infectious diseases. The microor-
ganisms that cause infectious diseases are called
pathogens.
Pathogens may also be called parasites, that is, they
live on another living organism called a host and
cause harm to the host. Some parasites cause diseases
that are fatal to the host, but many others live in a more
balanced way with their hosts. These cause illnesses
from which the host recovers, and the parasite often
survives long enough to be spread to other hosts.

Our introduction to microbiology will begin with a

brief description of the classification and naming of

microorganisms, followed by a discussion of normal

flora. We will then consider infectious diseases, types

of infection, and the spread and control of infection.

Last, we will describe the types of pathogens in more

detail and include the methods we have to treat and

control the diseases they cause. For a historical per-

spective on microbiology, see Box 22–1: The Golden

Age of Microbiology, and Box 22–2: Microbiology in

the 20th Century.

CLASSIFICATION OF

MICROORGANISMS

Bacteriaare very simple, single-celled organisms that

are found virtually everywhere. The natural habitats

of bacteria include freshwater, saltwater, soil, and

other living organisms. Most bacteria are not harmful

to us, and within their normal environments, they

have the vital role of decomposing dead organic mate-

rial and recycling their nutrients. However, a number

of bacteria cause human diseases, including strep

throat, pneumonia, and meningitis.

Virusesare not cells; they are even smaller and

498 An Introduction to Microbiology and Human Disease

BOX22–1 THE GOLDEN AGE OF MICROBIOLOGY

1881 Discovery that staphylococci cause infections.

1882 The cause of tuberculosis is found to be a

bacterium.

1883 Diphtheria is found to be caused by a bac-

terium.

1884 The bacteria that cause tetanus, typhoid

fever, and cholera are each isolated.

The first description of phagocytosis by white

blood cells is published.

1885 Louis Pasteur first uses his vaccine for rabies.

1890 Development of antitoxins to treat tetanus and

diphtheria.

1892 Discovery and demonstration of a virus.

1894 The bacterium that causes plague is discovered.

1897 Discovery of the cause of botulism.

Rat fleas are found to be vectors of plague.

1900 Walter Reed and his associates demonstrate

that yellow fever is caused by a virus and that

the vector is a mosquito.

Microorganisms were first seen in the 17th century

when simple microscopes were developed. The

roles of microorganisms, however, especially in rela-

tion to disease, were to remain largely unknown for

another two centuries. The years between 1875

and 1900 are sometimes called “The Golden Age of

Microbiology” because of the number and signifi-

cance of the discoveries that were made during this

time. The following is only a partial list.

1877 Robert Koch proves that anthrax is caused by a

bacterium.

1878 Joseph Lister first grows bacteria in pure cul-

ture.

The oil immersion lens for microscopes is

developed.

1879 The bacterium that causes gonorrhea is discov-

ered.

1880 Louis Pasteur develops a vaccine for chicken

cholera.