14．
The Spread of Disease
Traveling from one city to another can lead to the spread of disease.When people arrive in a
new place，they may bring in bacteria that （1） was not （not be） present in the past.The
people there have no natural protection （2） against these new bacteria，so they catch the
disease more easily.（3） Because it is unfamiliar，health workers may not identify it or stop it
from spreading.People with the disease may continue to have contact with others.In this way，the
bacteria travel from person to person through the population.This was （4） what happened，
for example，when soldiers travelled back home at the end of the First World War.These soldiers
brought with them the Spanish flu，and （5） it cost millions of lives.
Heating and cooling systems in buildings can also be a source of disease.Take old air
conditioners in windows as an example.They tend to collect dirt and water，which makes it easy
for bacteria （6） to grow （grow）.Then when the air conditioner （7） is turned
（turn） on，the bacteria will be blown into the home or office and make people sick.Bacteria
grow fast in the water of the cooling towers.They are then sent throughout the building with the air
conditioning and can affect anyone in the building.Scientists first recognized this problem in July
1976，by （8） that time 221 people had become seriously ill.
Pollution of the oceans can also be a factor in spreading disease.The pollution may be caused
by fertilizers that wash into rivers and then into the ocean，or by human waste （9） dumped
（dump）directly into the ocean with no processing.These pollutants result in the increased growth
of tiny plants that are called algae.They can form a thick mass in the water， （10） providing
（provide）a perfect environment for cholera，a fatal disease.
(1) was not; (2) against; (3) Because; (4) what; (5) it; (6) to grow; (7) is turned; (8) that; (9) dumped（10）providing