Can orchitis be caused by the flu?

Maybe: Orchitis is inflammation (redness, warmth, tenderness, swelling) of one or both testicles, generally caused by a viral or bacterial infection. Influenza virus (the flu) is one of the viruses that has been reported to occasionally cause orchitis as part of a generalized febrile illness.

Orchitis: Some viral pathogens such as the mumps virus can involve the testes. Influenza has not been linked with orchitis to the best of my knowlege. Most orchitis, however is bacterial.