<AREA SHAPE=RECT COORDS="177,124,190,139" HREF="!Tubercle bacilli in the air are inhaled deep into the tiniest tubes of the lung at the start of a primary infection.">
<AREA SHAPE=RECT COORDS="335,126,350,139" HREF="!Macrophages attack the invading bacilli. These cells may kill the bacteria or engulf them without killing them.">
<AREA SHAPE=RECT COORDS="493,125,507,139" HREF="!Other defending cells trap the remaining bacilli in hard lumps called tubercles. The trapped bacilli are harmless.">
<AREA SHAPE=RECT COORDS="176,282,190,296" HREF="!The bacilli break out of the tubercles if the body's defenses weaken. This marks the start of tuberculosis.">
<AREA SHAPE=RECT COORDS="335,283,350,297" HREF="!The bacilli multiply and invade surrounding tissue. Macrophages unsuccessfully attack the bacilli.">
<AREA SHAPE=RECT COORDS="492,284,507,297" HREF="!Multiplying bacilli break through the lung tube wall and invade a blood vessel. They may then spread.">
Most cases of tuberculosis begin with an infection deep in the lung. The top series of drawings shows how invading bacteria called <I>tubercle bacilli</I> cause a primary infection. The bottom drawings illustrate how tuberculosis can later develop from the primary infection.<NP>
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World Book diagram by Robert Demerest</P>
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