The discovery and rapid onset of the H1N1 flu, first termed swine flu, in the spring of 2009 caused the World Health Organization to declare a worldwide pandemic, reports MayoClinic.com. This flu looks genetically similar to swine flu viruses, but it passes easily among humans, thus the original name of swine flu. For several weeks in late 2009 to early 2010, the deaths attributed to pneumonia and influenza were above what would be seen in a normal flu season and therefore met the standard definition of an epidemic, reports the Centers for Disease Control and Prevention (CDC).

Allergy immunotherapy, also known as allergy shots or allergy immunization, treats allergy symptoms by retraining the immune system. Understanding how allergy symptoms develop will help you understand allergy shots. When your body gets exposed to an allergen, such as pollen, your body then produces an antibody called IgE reports the American Academy of Pediatrics in its book “Caring for Your Baby and Young Child.” This IgE then stimulates the body to release histamines and leukotrienes that cause the sneezing, coughing and running nose symptoms that are common with allergies.