The World Health Organization has added all of Sao Paulo state to its list of areas at risk for yellow fever.
That puts the megacity of Sao Paulo on the list and means that the organization is recommending that all international visitors to the state be vaccinated.
Tuesday's announcement comes as an outbreak is gathering steam in Brazil ahead of Carnival, a major draw for foreign tourists. The WHO says 11 human cases have been confirmed through last week and hundreds more found in monkeys.
Much of Brazil is considered at risk for yellow fever, but the coast was largely considered safe. Last year, however, Brazil saw an unusually large outbreak of the disease, including in areas not previously at risk. In response, Brazil rushed to vaccinate millions of people.