After completing her medical residency, Guinan joined a two-year training program with the Epidemic Intelligence Service of the CDC, and asked to work with the World Health Organization on smallpox eradication. Guinan spent five months with a team in Uttar Pradesh, India, where she worked to identify smallpox, and ring vaccinating those susceptible to the disease in the immediate area. Soon after Guinan returned to the U.S. to complete her program, Uttar Pradesh reached a zero infection rate. She states, "That experience changed my life, and I decided to go into public health."
After her EIS training, Guinan was accepted to an infectious disease fellowship at the University of Utah. There, she studied the herpes virus, focusing on oral herpes. Soon she found herself "an expert" on genital herpes after answering questions and lecturing about oral herpes. After many days of news interviews and calls from all over the world asking for help with genital herpes, Guinan decided to become the genital herpes expert everyone thought she was; she focused on women, who were not studied as thoroughly as men, at that time. She became known as "The Herpes Expert".
HIV/AIDS
In 1978, Guinan was asked to work with the Venereal Disease Control Division of the CDC. By 1981, when the CDC began investigating HIV/AIDS she was the only virologist in the STD unit. She was made a member of the AIDS task force, which was credited with "discovering" the AIDS epidemic. While investigating the epidemic, a newspaper columnist, Randy Shilts, often interviewed Guinan. In 1987, the book And the Band Played On was published, and included interviews with Dr. Guinan. In 1990, after becoming the first woman to be the Associated Director for Science at the CDC, Guinan was made the Assistant Director for Evaluation, Office of HIV/AIDS at the CDC, a position she would hold until 1995. In 1995–98, still with the CDC, she was the named the Chief of the , a community-based prevention and health promotion effort.
Nevada Chief State Health Officer
In 1998, Guinan became the Nevada State Chief Health Officer, the first woman to hold that position. When asked why she chose Nevada, she said that while states like New York and California had a solid base of public health practices, Nevada was just beginning to develop its plans; as such, it was a great opportunity to make a large impact on the health of Nevada's citizens. During her tenure she oversaw investigations regarding the Fallon cancer cluster, the Southern Nevada hepatitis C outbreak of 2008 linked to the re-use of equipment at the Endoscopy Center of Nevada, and the state's reaction to the 2009 H1N1 influenza A viruspandemic.
