Britannica Student Encyclopedia - 2010

outside big cities. Huge companies control

much of the world’s farmland.

Crops grow on large plots of land with

the help of irrigation and special chemicals.

Farmers raise healthy livestock with

the help of medicine. In poor countries,

however, many people still depend on

their own small farms to survive.

#More to explore

Industrial Revolution • Irrigation • Soil

AIDS

AIDS is a serious disease. The name

stands for acquired immunodeficiency

syndrome. Millions of people have died

from AIDS since the 1980s. Scientists

have not yet found a cure, but they can

treat the disease effectively.

Causes

Tiny particles called viruses cause many

diseases. The particular virus that causes

AIDS is called human immunodeficiency

virus (HIV). HIV enters the body

through contact with infected blood or

other body fluids. Coughing, sneezing,

and handshaking do not spread the

virus.

HIV kills helper T cells in the body.

Helper T cells are a type of white blood

Some farmers use special irrigation systems

to water their crops.

The AIDS Memorial Quilt is displayed in

Washington, D.C. The quilt honors people

who have died of the disease.

BRITANNICA STUDENT ENCYCLOPEDIA AIDS 49

 

cell. They are part of the body’s immune

system, which fights off disease. The

body loses its ability to fight off disease

when its helper T cells die.

Symptoms

A new HIV infection often causes symptoms,

or signs, that are similar to those

of influenza (flu). They usually last only

a week or two. After they clear up, an

infected person may show no signs of

illness for years. However, such a person

can still pass the virus to other people.

Because of their weak immune systems,

most HIV-infected people eventually

come down with other infections. A

person who has these infections is said

to have AIDS.

The most common infections include

tuberculosis and a certain type of

pneumonia. People with AIDS also

frequently develop a cancer called Kaposi

sarcoma. The AIDS virus may also attack

the nervous system and cause brain and

eye damage. Death often follows.

Prevention and Treatment

People can prevent HIV infection. The

main way to do this is to keep out of

contact with possibly infected blood or

body fluids.

If a person is infected, certain drugs can

stop HIV from doing damage. Drug

treatment has added years to the lives of

many people with HIV infections.

#More to explore

Disease, Human • Immune System

• Virus

Ailey, Alvin

Alvin Ailey was a famous modern

dancer. He was also a highly respected

choreographer—a person who creates

dances and directs dancers. His dance

company toured around the world,

bringing recognition to African Americans

in dance.

Ailey was born in Rogers, Texas, on

January 5, 1931. His family moved to

Los Angeles when he was 12. Ailey

began to study modern dance at the

Lester Horton Dance Theater in 1949.

He became the company’s director and

choreographer in 1953.

In 1954 Ailey moved to New York City,

where he studied with modern dance

pioneer Martha Graham and others. He

created the Alvin Ailey American Dance

Theater in 1958. The troupe performed

many kinds of works, but Ailey particularly

wanted to bring African American

culture to modern dance.

Members of the Alvin Ailey American

Dance Theater rehearse a scene from the

dance called Revelations.

AIDS exists

everywhere in

the world.

However, the

disease is a

particularly

big problem in

parts of

Africa.

50 Ailey, Alvin BRITANNICA STUDENT ENCYCLOPEDIA

 

In 1958 Ailey achieved his first big success

as a choreographer with Blues Suite.

One of his most famous dances is Revelations

(1960), which is set to African

American spirituals. His works combine

ballet and modern dance.

In 1965 Ailey retired from dancing, but

he continued choreographing. He died

in New York City on December 1,

1989. His company remained in operation

after his death.

#More to explore

Dance • Graham, Martha

Air

All living things need air to breathe.

Animals and plants would die without

it. Air also holds airplanes and kites

above the ground, and it is used to

inflate tires and balloons. Air surrounds

Earth as the atmosphere. Air has no

color, taste, or odor. Although air is

invisible, people can hear air when the

wind howls. And they can feel air brush

against their skin when the wind blows.

Air consists of a mixture of gases. These

gases consist of tiny particles called molecules.

They are so small that they cannot

be seen. Molecules are combinations

of different chemical elements. In air the

most common of these elements are

nitrogen and oxygen. Oxygen is an

important component of air because

humans and other animals need it to

survive. In addition to nitrogen and

oxygen, air consists of small amounts of

the gases argon, neon, helium, and

methane. Besides gases, air also contains

water vapor, dust, pollen, and bacteria.

The water vapor in air is the source of

rain and snow.

The oxygen in air comes from plants.

Through a process called respiration, or

breathing, humans and other animals

take in oxygen and force out carbon

dioxide. Carbon dioxide is a molecule

made of oxygen and carbon. Plants take

in the carbon dioxide and use it to make

food. This is part of the process known