According to the county's department of public health, the child was under 5 and was "severely immunocompromised."
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It's the county's first death of a child under 5 related to the respiratory virus that often causes colds but can be deadly for some.
No other information about the child was released.
"The loss of this young child underscores the importance of heightened awareness and protective measures, especially for vulnerable populations including older adults, pregnant people, and infants," said Dr. Michael Sequeira, San Bernardino County's Health Officer.
Last week, health officials in neighboring Riverside County said an infant who had died earlier in the month tested positive for RSV.
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The child in that case, who wasn't a year old yet, died at the hospital after suffering from "respiratory difficulties."
RSV can spread when someone with the virus sneezes or coughs, through close contact with someone who is sick, or by touching infected surfaces and then touching your face without washing your hands.
People with RSV infection typically have fever, cough, runny nose, wheezing-and in very young infants-increased irritability and difficulty breathing.