Between late 2008 and the middle of 2009 an outbreak of sylvatic yellow fever caused the deaths of seven people and more than 2,000 howler monkeys in the state of Rio Grande do Sul, southern Brazil. The disease outbreak spread panic among the local human population. Fear of the disease and misinformation disseminated by the media about the role of howler monkeys in the yellow fever cycle led inhabitants of several regions to illegally kill these already regionally threatened primates. In this talk I will make the case that howler monkeys are our allies in detecting the spread of the yellow fever virus by playing an important role as sentinels. The howlers did not cause the re-emergence of this African infectious disease nor did they promote its transmission or its rapid spread through the highly fragmented landscape of the state of Rio Grande do Sul. I also will explain the role of howler monkeys, humans and mosquitoes in the yellow fever cycle, and describe an outreach campaign 'Protect our Guardian Angels' that I launched to inform the public and the media that protecting howler monkeys is a matter of public health. The campaign was successful in getting the support of educational, scientific, governmental (health- and environment-related) and religious institutions, and NGOs, and was effective in changing how the news media presented scientific information to the public.