According to the U.S. Centers for Disease Control and Prevention (CDC), 5 to 20 percent of Americans contract the influenza (flu) virus each year. Flu produces a spectrum of disease, ranging from mild, self-limited respiratory disease to life-threatening pneumonia. Flu primarily affects the immune system, the respiratory system and the digestive tract.
The immune system fights infection caused by the flu and other viruses, bacteria, fungi and parasites. According to "Harrison’s Principles of Internal Medicine," cytokines are a kind of chemical distress call issued by T-cells, B-cells and natural killer cells of the immune systems in response to influenza infection. Most of the systemic symptoms such as fever, muscle pain and headache that are associated with infections actually reflect the release of these cytokines by the immune system.
Cytokines cue the hypothalamus in the brain to raise body temperature, which slows down or inactivates enzymes involved in viral replication. Cytokines also activate receptors in the blood vessels of the brain--causing headache--and trip pain fibers in muscles, accounting for the sensation of non-specific muscle pain and soreness. Cytokine release is proportionate to immune system response. Therefore, as the infection subsides, so do systemic symptoms.
According to the NIH, influenza is primarily a respiratory virus that produces both upper- and lower-respiratory symptoms. Lower-respiratory symptoms include cough and breathing problems. Upper-respiratory symptoms include sore throat, runny nose and congestion. Experimental models of influenza infection reveal that the virus induces death in the cells lining the respiratory tract. These dead cells are shed, enabling the virus to infect progressive cell layers. Respiratory symptoms of influenza are proportional to the “dose” of virus a person is infected with. Viruses also replicate using a person’s own cells like a factory. In people with delayed or deficient immune response, small initial doses can produce big effects due to unchecked growth.
Seasonal influenza is associated with a lack of appetite. This is believed to be a secondary effect of cytokines on the appetite centers of the brain. In addition, respiratory symptoms such as sore throat and runny nose diminish the desire to eat by making swallowing painful and producing nausea due to post-nasal drip.
Seasonal influenza is rarely associated with other gastrointestinal symptoms such as vomiting or diarrhea. However, the novel H1N1 or swine flu also appears to be different in this respect. According to a 2009 report in the "New England Journal of Medicine" by the Novel Swine-Origin Influenza A (H1N1) Virus Investigation Team, vomiting, diarrhea or both was reported by nearly 40 percent of people with laboratory-confirmed swine flu. Compared to the effects of swine flu on other body systems, these symptoms were mild and short-lived.