Diabetes impacts on the whole body, including the immune system. As such, diabetes can make some people more susceptible to common infections, while badly controlled diabetes can further increase this susceptibility.
Good control is critical not only for common infections, but for the more serious conditions regarded as having an increased risk for diabetics.
For flu there is an annual vaccination programme that all diabetics are entitled too, this/last year there is also the swine flu vaccine.
A vaccination against pneumonia is also available for diabetics, to stop chest infections from becoming worse.