Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae (the influenza viruses), that affects birds and mammals. The most common symptoms of the disease are chills, fever, sore throat, muscle pains, severe headache, coughing, weakness/fatigue and general discomfort. Sore throat, fever and coughs are the most frequent symptoms. In more serious cases, influenza causes pneumonia, which can be fatal, particularly for the young and the elderly. Although it is often confused with other influenza-like illnesses, especially the common cold, influenza is a much more severe disease than the common cold and is caused by a different type of virus. Influenza may produce nausea and vomiting, particularly in children, but these symptoms are more common in the unrelated gastroenteritis, which is sometimes called "stomach flu" or "24-hour flu".

Typically, influenza is transmitted through the air by coughs or sneezes, creating aerosols containing the virus. Influenza can also be transmitted by direct contact with bird droppings or nasal secretions, or through contact with contaminated surfaces. Airborne aerosols have been thought to cause most infections, although which means of transmission is most important is not absolutely clear. Influenza viruses can be inactivated by sunlight, disinfectants and detergents. As the virus can be inactivated by soap, frequent hand washing reduces the risk of infection.

In 2009, an outbreak of Swine Flu (H1N1) occurred worldwide. The virus that causes the disease originally only jumped from live pigs to humans who had direct contact with them, then evolved so that it could be passed from human to human. The virus is a “reassortant,” meaning a mix of genes from swine, bird, and human flu viruses.

H5N1, or "bird flu," is spread when wild birds pass the virus along birds that are raised for human consumption, for example, chicken, turkey, or ducks. The wild birds do not seem to exhibit symptoms from the virus, but the livestock that contract the virus do get sick. The virus is then passed to humans when they consume the tainted livestock. Human bodies do not have antibodies to bird flu and therefore most people must be hospitalized to fight the virus.

9-tetrahydrocannabinol (Δ9-THC) has been widely established as a modulator of host immune responses" writes the research team of the study "Modulation of Airway Responses to Influenza A/PR/8/34 by Δ9-Tetrahydrocannabinol in C57BL/6 Mice" at Michigan State University. "Accordingly, the objective of the present study was to examine the effects of Δ9-THC on the immune response within the lungs and associated changes in the morphology of the bronchiolar epithelium after one challenge with a nonlethal dose of the influenza virus A/PR/8 (PR8)." C57BL/6 mice were treated by an oral tube with Δ9-THC and/or vehicle (corn oil) for 5 consecutive days. On the 3rd day, 50 plaque-forming units of PR8 and/or vehicle (saline) were introduced intranasally 4 hours before the Δ9-THC exposure. The mice were killed 7 and 10 days post infection. The levels at which mRNA levels of viral hem agglutinin 1 (an allergic reaction type occurrence where cells become more compacted together to prevent foreign materials entering them) were increased in a dose-dependent manner with Δ9-THC treatment. The degree of inflammation and virus-induced mucous cell metaplasia (replacement of one differentiated cell type with another mature differentiated cell type) was reduced in Δ9-THC-treated mice by 10 days post infection.