Because the human body provides an ideal environment for many microbes, they
try to pass your skin barrier and enter. Your immune system is a bodywide
network of cells, tissues, and organs that has evolved to defend you against
such "foreign" invasions.
The proper targets of your immune system are infectious organisms--bacteria
such as these streptococci; fungi (this one happens to be Candida, the cause of
yeast infections); parasites, including these worm-like microbes that cause
malaria; and viruses such as this SARS virus.
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