Re: Why do we get intense body aches when we have the flu?

You are correct in that it is difficult to find an answer to your question on the Web. I was trying to find a good link for you, but found none. Here is a basic summary of what causes the symptoms encountered during the flu.

Common flu symptoms include fever, sore throat, muscle aches, shivers and cold sweats, etc. These symptoms are not caused by the virus itself but rather by our bodies' response to the the virus. The immune system mounts a two-fold response to the virus. The humoral response (antibody- mediated) produces antibodies that bind to influenza receptors, preventing further infection of uninfected cells. This is the primary means by which the influenza infection is arrested. The cellular response acts by destroying viral infected cells. This is where the nasty symptoms come from. T cells and macrophrages attack mucosal cells that have been altered by the virus and destroy them. The cells produce chemicals known as cytokines and interleukins that either destroy the abnormal cell or recruit other immune cells into the area that is infected. This cell- mediated response is intense and results in increased concentrations of the proinflammatory cytokines tumor necrosis factor (TNF), interferon gamma (IFN), and interleukin 6 (IL-6), which have been shown to parallel disease severity in clinical studies.(Hayden FG, Treanor JJ, Betts RF, et al. Safety and efficacy of the neuraminidase inhibitor GG167 in experimental human influenza. JAMA 1996;275:195-199.)

The specific cause of body aches (myalgia) are the interleukins, which are primarily produced by the macrophages. These interleukins cause pain, body aches, hypersensitivity of the nerves, etc. Some of the interleukins (such as IL-2) cause the fever seen during the flu, and probably the headaches. Since these interleukins are produced in response many different kinds of bacterial and viral infections, symptoms such as fever, myalgia, and headaches can be seen in infections by many different etiologic agents.