According to a statement issued by the county Department of Public Health, the patient is "an adult resident" who recently traveled.
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"They are symptomatic but doing well and isolating away from others," read the statement.
Health officials said contact tracing is being performed to identify anyone who was in close proximity to the patient, and "post-exposure prevention'' efforts have been enacted for close contacts.
The first presumed case in L.A. County was announced on June 2.
What is monkeypox? What to know about virus, symptoms, spread
What to know after monkeypox case confirmed in US
Monkeypox is known to spread when there is close physical contact with an infected person, their clothing or bedsheets.
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Most monkeypox patients experience only fever, body aches, chills and fatigue. People with more serious illness may develop a rash and lesions on the face and hands that can spread to other parts of the body. No deaths have been reported in the current outbreak beyond Africa.
Monkeypox is related to smallpox, but has milder symptoms. After smallpox was declared eradicated in 1980, countries suspended their mass immunization programs, a move that some experts believe may be helping monkeypox spread now, since there is now little widespread immunity to related diseases. Smallpox vaccines are also protective against monkeypox.
Answers to frequently asked questions about monkeypox are available on the Department of Public Health's website.
The Associated Press contributed to this report.