Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

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Pathogens and Infection
chapter
23
Infectious diseases currently cause about one-quarter of all human deaths worldwide,
more than all forms of cancer combined and second only to cardiovascular
diseases. In addition to the continuing heavy burden of ancient diseases such as
tuberculosis and malaria, newer infectious diseases continually emerge. The current
pandemic (worldwide epidemic) of AIDS (acquired immune deficiency syndrome),
was first clinically observed in 1981 and has since caused more than 35
million deaths worldwide. Moreover, some diseases long thought to result from
other causes are now recognized to be associated with infections. Most gastric
ulcers, for example, are caused not by stress or spicy food, but by infection of the
stomach lining by the bacterium Helicobacter pylori.
The burden of infectious diseases is not spread equally across the planet.
Poorer countries and communities suffer disproportionately, often due to poor
public sanitation and health systems. Some infectious diseases, however, occur
primarily or exclusively in industrialized communities: Legionnaire’s disease, for
example, a bacterial infection of the lungs, commonly spreads through air-conditioning
systems.
Since the mid-1800s, physicians and scientists have struggled to identify the
agents—collectively called pathogens—that are capable of causing infectious
diseases. More recently, the advent of microbial genetics and molecular cell biology
has greatly enhanced our understanding of the causes and mechanisms of
infectious diseases. We now know that pathogens frequently exploit the attributes
of their host’s cells in order to infect them. This understanding can give us new
insights into normal cell biology, as well as strategies for treating and preventing
infectious diseases.
Although pathogens are understandably a focus of attention, only a relatively
small fraction of the microbial species we encounter are pathogens. Much of the
biomass of the Earth is made up of microbes, and they produce everything from
the oxygen we breathe to the soil nutrients we use to grow food. Even those species
of microbes that colonize the human body do not generally cause disease.
The collective of microorganisms that reside in or on an organism is called the
microbiota. Many of these microbes have a beneficial effect on the health of the
organism, assisting its normal development and physiology.
In this chapter, we give an overview of the different kinds of pathogens, as well
as those microorganisms that colonize our body without causing trouble. We then
discuss the cell biology of infection—the molecular interactions between pathogens
and their host. In Chapter 24, we consider how our innate and adaptive
immune systems collaborate to defend us against pathogens.
In This Chapter
INTRODUCTION TO PATHOGENS
AND THE HUMAN MICROBIOTA
CELL BIOLOGY OF INFECTION
INTRODUCTION TO PATHOGENS AND THE HUMAN
MICROBIOTA
We normally think of pathogens as hostile invaders, but a pathogen, like any other
organism, is simply exploiting an available niche in which to live and procreate.
Living on or in a host organism is a very effective strategy, and it is possible that
every organism on Earth is subject to some type of infection (Figure 23–1). A
human host is a nutrient-rich, warm, and moist environment, which remains at a