Legionnaires’ disease WORLD OF MICROBIOLOGY AND IMMUNOLOGY344•
notice, thinking that such magnification from a single lens
microscope could only be a hoax. However, in 1676, when he
sent the Society the news that he had discovered tiny one-
celled animals in rainwater, the interest of member scientists
was piqued. Following Leeuwenhoek’s specifications, they
built microscopes of comparable magnitude and confirmed his
findings. In 1680, the Society unanimously elected
Leeuwenhoek as a member.
Until this time, Leeuwenhoek had been operating in an
informational vacuum; he read only Dutch and, conse-
quently, was unable to learn from the published works of
Hooke and Malpighi (though he often gleaned what he could
from the illustrations within their texts). As a member of the
Society, he was finally able to interact with other scientists.
In fact, the news of his discoveries spread worldwide, and he
was often visited by royalty from England, Prussia, and
Russia. The traffic through his laboratory was so persistent
that he eventually allowed visitors by appointment only.
Near the end of his life, Leeuwenhoek had reached near-leg-
endary status and was often referred to by the local towns-
folk as a magician.
Amid the attention, Leeuwenhoek remained focused
upon his scientific research. Specifically, he was interested in
disproving the common belief in spontaneous generation, a
theory proposing that certain inanimate objects could gener-
ate life. For example, it was assumed that moldand maggots
were created spontaneously from decaying food.
Leeuwenhoek succeeded in disproving spontaneous genera-
tion in 1683, when he discovered bacteria cells. These tiny
organisms were nearly beyond the resolving power of even
Leeuwenhoek’s remarkable equipment and would not be seen
again for more than a century.
Leeuwenhoek created and improved upon new lenses
for most of his long life. For the forty-three years that he was
a member of the Royal Society, he wrote nearly 200 letters
that described his progress. However, he never divulged the
method by which he illuminated his specimens for viewing,
and the nature of that illumination is still somewhat of a mys-
tery. Upon his death on August 30, 1723, Leeuwenhoek willed
twenty-six of his microscopes, a few of which survive in
museums, to the British Royal Society.See alsoBacterial growth and division; Bacterial kingdoms;
Bacterial membranes and cell wall; Bacterial movement;
History of microbiology; Microscope and microscopyLLegionnaires’ diseaseEGIONNAIRES’ DISEASE
Legionnaires’ disease is a type of pneumoniacaused by
Legionellabacteria. The bacterial species responsible for
Legionnaires’ disease is L. pneumophila.Major symptoms
include fever, chills, muscle aches, and a cough that is initially
nonproductive. Definitive diagnosis relies on specific labora-
tory tests for the bacteria, bacterial antigens, or antibodies pro-
duced by the body’s immune system. As with other types of
pneumonia, Legionnaires’ disease poses the greatest threat to
people who are elderly, ill, or immunocompromised.Legionellabacteria were first identified as a cause of
pneumonia in 1976, following an outbreak of pneumonia
among people who had attended an American Legion conven-
tion in Philadelphia, Pennsylvania (the bacterium’s name was
derived from this group’s name). This outbreak prompted fur-
ther investigation into Legionellaand it was discovered that
earlier unexplained pneumonia outbreaks were linked to the
bacteria. The earliest cases of Legionnaires’ disease were
shown to have occurred in 1965, but samples of the bacteria
exist from 1947.
Exposure to the Legionellabacteria does not necessar-
ily lead to infection. According to some studies, an estimated
5–10% of the American population show serologic evidence
of exposure, the majority of whom do not develop symptoms
of an infection. Legionellabacteria account for 2–15% of the
total number of pneumonia cases requiring hospitalization in
the United States.
There are at least 40 types of Legionellabacteria, half of
which are capable of producing disease in humans. A disease
that arises from infection by Legionellabacteria is referred to
as legionellosis. The L. pneumophilabacterium, the root cause
of Legionnaires’ disease, causes 90% of legionellosis cases.
The second most common cause of legionellosis is the L. mic-
dadeibacterium, which produces the Philadelphia pneumonia-
causing agent.
Approximately 10,000–40,000 people in the United
States develop some type of Legionnaires’ disease annually.
The people who are the most likely to become ill are over age- The risk is greater for people who suffer from health con-
ditions such as malignancy, diabetes, lung disease, or kidney
disease. Other risk factors include immunosuppressive ther-
apy and cigarette smoking. Legionnaires’ disease has occurred
in children, but typically, it has been confined to newborns
receiving respiratory therapy, children who have had recent
operations, and children who are immunosuppressed. People
with HIV infection and AIDSdo not seem to contract
Legionnaires’ disease with any greater frequency than the rest
of the population, however, if contracted, the disease is likely
to be more severe compared to other cases.
Cases of Legionnaires’ disease that occur in conjunction
with an outbreak, or epidemic, are more likely to be diagnosed
quickly. Early diagnosis aids effective and successful treat-
ment. During epidemic outbreaks, fatalities have ranged from
5% for previously healthy individuals to 24% for individuals
with underlying illnesses. Sporadic cases (that is, cases unre-
lated to a wider outbreak) are harder to detect and treatment
may be delayed pending an accurate diagnosis. The overall
fatality rate for sporadic cases ranges from 10–19%. The out-
look is bleaker in severe cases that require respiratory support
or dialysis. In such cases, fatality may reach 67%.
Legionnaires’ disease is caused by inhaling Legionella
bacteria from the environment. Typically, the bacteria are dis-
persed in aerosols of contaminated water. These aerosols are
produced by devices in which warm water can stagnate, such
as air-conditioning cooling towers, humidifiers, shower heads,
and faucets. There have also been cases linked to whirlpool
spa baths and water misters in grocery store produce depart-
ments. Aspiration of contaminated water is also a potential
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