The county has three confirmed cases since June 3. Two people needed hospital treatment, and there are 19 more suspected cases. Most have been traced to one encampment.
"We're really pursing this situation aggressively because of the number of cases as well as the setting and making sure that this doesn't spread further," said Monika Roy, assistant health officer of Santa Clara County Health Dept.
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Shigella is a highly contagious bacteria that can cause diarrhea and sometimes severe disease.
It can be spread by eating or drinking something contaminated with the bacteria - from a shared surface or directly from someone who is infected.
The infection clears itself without treatment for most healthy people.