In the spring of 2019, Peruvian neurologists saw a significant increase in the number of patients with Guillain-Barre syndrome (GBS). This rare disorder occurs when a person’s immune system damages the nerves, causing muscle weakness and sometimes paralysis. Although GBS typically only affects 3,000-6,000 people annually in the United States (or about 1 in 100,000), Peruvian doctors documented hundreds of cases between May and July 2019 and deemed it an outbreak.