able to bind; this leads to the virus being able to evolve and mutate into different forms. Influenza A is responsible for infecting swine, and was first identified in 1918. Pigs have often been seen as "mixing vessels", which help to change and evolve strains of disease that are then passed on to other mammals, such as humans. Humans Direct transmission of a swine flu virus from pigs to humans is occasionally possible (zoonotic swine flu). In all, 50 cases are known to have occurred since the first report in medical literature in 1958, which have resulted in a total of six deaths. Of these six people, one was pregnant, one had leukemia, one had Hodgkin's lymphoma and two were known to be previously healthy. One of these had unknown whereabouts. Despite these apparently low numbers of infections, the true rate of infection may be higher, since most cases only cause a very mild disease, and will probably never be reported or diagnosed. According to the United States Centers for Disease Control and Prevention (CDC), in humans the symptoms of the 2009 "swine flu" H1N1 virus are similar to influenza and influenza-like illness in general. Symptoms include fever, cough, sore throat, watery eyes, body aches, shortness of breath, headache, weight loss, chills, sneezing, runny nose, coughing, dizziness, abdominal pain, lack of appetite, and fatigue. The 2009 outbreak also had an increased percentage of patients reporting diarrhea and vomiting. The 2009 H1N1 virus is not zoonotic swine flu, as it is not transmitted from pigs to humans, but from person to person through airborne droplets. Because these symptoms are not specific to swine flu, a differential diagnosis of probable swine flu requires not only symptoms, but also a high likelihood of swine flu due to the person's recent and past medical history. For example, during the 2009 swine flu outbreak in the United States, the CDC advised physicians to "consider swine influenza infection in the differential diagnosis of patients with acute febrile respiratory illness who have either been in contact with persons with confirmed swine flu, or who were in one of the five U.S. states that have reported swine flu cases or in Mexico during the seven days preceding their illness onset." A diagnosis of confirmed swine flu requires laboratory testing of a respiratory sample (a simple nose and throat swab). The most common cause of death is respiratory failure.