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Immunity and Disease
Diseases
Disease Through History
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Thousands of years ago, a doctor might have treated a
disease by scraping a person’s skull with a rock until a hole
formed. The doctor made the hole in the skull to let the
illness escape. Archaeologists know this because they have
discovered skulls with smooth holes in them. Bone growth
around the holes shows that people lived after these holes
were made. This treatment may sound strange today.
However, it was an accepted treatment for disease thousands
of years ago.
Today we know that many diseases are caused by bacteria
and viruses. Disease-causing agents, such as bacteria and viruses,
are called pathogens. Pathogens have always caused illnesses.
However, it has been only during the last few hundred years
that scientists have understood the relationship between
pathogens and diseases. Before then, little was known about
disease and immunity, and superstitions were common.
Early Research on Diseases
During the eighteenth and nineteenth centuries, doctors
learned a lot about the causes and treatment of diseases. The
research and experiments performed by scientists such as
Edward Jenner, John Snow, and Anton van Leeuwenhoek
saved many lives.
First Vaccination
In 1796, Edward Jenner developed the first vaccination. A
vaccination is a procedure that helps the body defend itself
against disease. Jenner knew that women who milked cows
often got a mild disease called cowpox.
However, these women did not get smallpox, a deadly
disease related to cowpox. Jenner, who was a doctor, cut the
arm of a young boy and placed pus from a cowpox sore in
the cut. Two weeks later, Jenner infected the boy with
smallpox. Like the women who milked the cows, the boy
never got sick from smallpox. Although the smallpox vaccine
saved many lives, people did not understand why or how it
worked.
Connecting Disease with a Source
Cholera death
Water pump
Contaminated water pump
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
In the mid-1800s, people realized that there was a
connection between pathogens and disease. Cholera is a
disease of the intestinal tract caused by a bacterium. Dr. John
Snow connected the disease to a bacterium he found in
water. He did this by tracking the origin of a cholera
outbreak on a map similar to the one below. Using the map,
Snow tracked the outbreak to a single water pump in the
city of London. He had the pump closed. Almost
immediately, the number of new cholera cases decreased.
When Snow looked at the water, he saw a microscopic
organism that he thought caused the disease. Not everyone
agreed with Snow, but people were beginning to think that
pathogens did exist.
The Development of Microscopes
One of the reasons people were slow to accept the idea of
pathogens is because people could not see the pathogens.
However, the development of microscopes changed that. In
the late 1600s, Anton van Leeuwenhoek (LAY vun hook)
made one of the first microscopes. Using the microscope,
he observed pond water and saw moving organisms. Van
Leeuwenhoek had discovered bacteria. Because he did not
share how he made the lenses, bacteria were not observed
again until the nineteenth century.
Connecting Bacteria to Infections
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
As first, scientists thought that wounds caused bacteria
to appear. Louis Pasteur experimented with bacteria in the
mid-1800s. He realized that this idea was backwards. Instead,
bacteria caused the tissue in the wound to decay. He found
that he could kill bacteria in boiling liquids. Pasteurization is
the process in which a food is heated to a temperature that kills most
harmful bacteria. The pasteurization process used today is based
on the work of Pasteur.
Joseph Lister used Pasteur’s discoveries to make surgery safer
for patients. He found that a substance called carbolic acid
killed bacteria. He developed a system of spraying carbolic acid
throughout an operating room during surgery. Infection and
death from surgery decreased greatly. In the late 1800s, doctors
improved on Lister’s idea. They used carbolic acid to sterilize
tools before surgery. They also used steam to sterilize the
clothes and linens used during surgery. Sterilization
completely kills the bacteria.
Discovering Disease Organisms
In 1867, Robert Koch developed a set of rules that could
be used to find out if certain bacteria caused an illness. The
research based on these rules convinced most scientists that
bacteria were disease-causing pathogens. Koch’s rules are
described in the following figure.
1 The bacterium must be
2 The bacterium must
found in all organisms
reproduce in the lab.
suffering from the disease,
but not in healthy organisms.
3 A sample of the newly grown
pathogen must cause the
illness when injected into a
healthy animal.
4 When the suspected
pathogen is removed from
the infected animal and
grown in the lab, it must be
identical to the original
pathogen.
Human Diseases and the Pathogens
That Cause Them
Pathogens
Diseases Caused
Viruses
flu, colds, chickenpox, and AIDS
Bacteria
ear infections, strep throat, pneumonia, meningitis,
whooping cough, and syphilis, a sexually transmitted
disease
Fungi
athlete’s foot, ringworm, and yeast infections
Protists
malaria, African sleeping sickness, and dysentery
Bacteria aren’t the only pathogens that cause disease.
Viruses, fungi, and protists can also cause disease.
The table above shows diseases caused by different
pathogens. Pathogens can be passed through water and food
and carried by insects. They can also be passed directly among
people by physical contact, sneezing, coughing, or exchange
of bodily fluids. Some pathogens require a host to reproduce.
Types of Diseases
Infectious Diseases
Diseases caused by pathogens that can be transmitted from one
person to another are infectious diseases. For example, flu and
cold viruses can be passed through direct contact, such as by
shaking hands.
The human immunodeficiency virus (HIV) can pass to
others through the exchange of blood or other bodily fluids.
HIV causes acquired immunodeficiency syndrome (AIDS), a
disease that attacks the body system that fights pathogens.
The protist that causes malaria is transferred by a vector.
A vector is a disease-carrying organism that does not develop the
disease. The vector for malaria is a certain type of mosquito.
The mosquito bites an animal that has the protist in its
blood. The protist enters the mosquito’s saliva, but the
mosquito does not develop malaria. When the mosquito bites
another animal, the protist moves into that animal’s blood.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Have you ever heard someone say, “I caught a cold”? The
common cold is contagious. This means that the pathogens
can be passed from person to person. Not all diseases are
caused by pathogens. Some diseases are the result of inherited
traits. Other diseases can be caused by factors such as your
environment and the choices you make about diet, exercise,
and sleep.
Noninfectious Diseases
A disease that cannot pass from person to person is a noninfectious
disease. For example, you cannot catch lung cancer from a
person who has lung cancer. Pathogens do not directly cause
noninfectious diseases. There are two common causes of
noninfectious diseases:
• genetics, or traits inherited in your DNA from your
biological parents, and
• environmental conditions, including lifestyle choices.
A person who has a genetic trait might develop a
noninfectious disease because of environmental conditions.
It is the combination of genetics and environment that
causes the disease to develop.
Childhood Diseases Noninfectious diseases that affect children
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
are mainly a result of genetics. Cystic fibrosis is a genetic
disease. It causes the body to produce mucus that is thicker
than normal. This affects breathing and other body functions.
The gene that causes cystic fibrosis is recessive. This means a
child must inherit this recessive gene from each parent to be
affected by cystic fibrosis. The parents, called carriers, do not
have to have the disease.
For many children with genetic disorders, environmental
conditions can make the disease worse. A poor diet, air
pollution, and lack of exercise make the symptoms of cystic
fibrosis worse.
Other Diseases Many noninfectious diseases that affect adults
are mainly a result of environmental causes and life choices.
For example, an unhealthful diet, obesity, a lack of regular
exercise, and smoking cause most cases of heart disease.
Osteoporosis is a disease in which bones become weak and
less dense. People inherit a tendency to develop osteoporosis.
However, years of poor lifestyle choices, such as an
unhealthful diet, lack of calcium and vitamin D, smoking,
and a lack of exercise can lead to weakened bones.
Cancer Tumors form when cells reproduce uncontrollably.
Cancer is a disease in which cells reproduce uncontrollably without the
usual signals to stop. For example, lung-cancer tumors form in
the lungs. The tumors decrease normal lung function. People
can inherit forms of genes that make them more likely to
develop lung cancer. But if they are not exposed to harmful
environmental conditions such as poor air quality and
smoking, they might not develop lung cancer.