HUMAN BIOLOGY

190 ChapteR 10

pathogens and Cancer in the respiratory system

resistant strains of M. tuberculosis have made treatment

much more challenging. Untreated TB can be fatal.

Lung cancer (Figure 10.22) kills more people than

any other cancer. Long-term tobacco smoking is the over-

whelming risk factor. In the body, some compounds in

tobacco smoke and coal tar are converted to carcinogens

(cancer-causing substances). They trigger genetic damage

leading to lung cancer. Other risks are exposure to asbes-

tos, radiation, and industrial chemicals such as arsenic.

Recently the incidence of lung cancer has fallen among

men but risen among women. This shift is thought to be

due to a rise in the relative number of female smokers sev-

eral decades ago. Warning signs include cough, shortness

of breath, chest pain, bloody phlegm, unexplained weight

loss, and frequent respiratory infections or pneumonia.

Four types of lung cancer account for 90 percent of

cases. About one-third of lung cancers are squamous

cell carcinomas in squamous epithelium in the bron-

chi. About 48 percent are either adenocarcinomas or

large-cell carcinomas. The most aggressive type, small-

cell carcinoma, kills most patients within 5 years.

Inhaled viruses, bacteria, or fungi all can infect respiratory

organs. A dry cough, chest pain, and shortness of breath

are symptoms of pneumonia. The infection inflames lung

tissue (Figure 10.21A), and then fluid (from edema) builds

up in the lungs and makes breathing difficult.

Strains of Streptococcus pneumoniae can cause pneu-

monia and other infections. This bacterium often causes

outbreaks of illness among children at day-care centers.

Penicillin or some other antibiotic is the usual treatment

for bacterial pneumonia. Unfortunately, today half of all

strains of S. pneumoniae are antibiotic-resistant.

Sometimes the trigger for pneumonia is influenza, in

which an infection that began in the nose or throat spreads

to the lungs. There are many flu viruses, but several have

made headlines recently. One is the H1N1 flu virus men-

tioned in Section 9.11. In 2014 a particularly virulent form

of this microbe hit the United States. Infections sometimes

led to such rapid “flooding” of the lungs with fluid that the

victims died within a few days. (Many were young or

middle-aged people who hadn’t been vaccinated with the

preventative vaccine.) Another virus causes SARS—severe

acute respiratory syndrome. A 2003 outbreak of SARS in

China eventually traveled around the globe (Figure 10.21B).

So-called bird flu, or avian influenza, is caused by the H5N1

virus. To date it has killed about 380 people, nearly all of

whom had close contact with infected wild birds. Health

authorities worry that the virus may

mutate in a way that allows human-

to-human transmission.

Tuberculosis (TB) is a lung

infection caused by the bacterium

Mycobacterium tuberculosis (left). It

starts with flulike symptoms but

eventually can destroy patches of

lung tissue and can spread to other

parts of the body. Antibiotics usu-

ally can cure TB, but newer drug-

Figure 10.22 Here a colored X-ray reveals a

malignant lung tumor (purple and orange).

Mycobacterium

tuberculosis

CAMR/A. Barry Dowsett/

Science Source

Figure 10.21 Many pathogens can infect the lungs. a Color-enhanced X-ray showing lung inflammation

(orange) caused by pneumonia. B A health care worker in China wears protection against the SARS virus,

which causes a form of influenza.

A: Du Cane Medical Imaging Ltd./Science Source;

B: © WHO, Pierre-Michel Virot, photographer

a B

10.8

Zephyr/Science Source

10.9

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