WordNet (r) 3.0 (2006):

flu
    n 1: an acute febrile highly contagious viral disease [syn:
         influenza, flu, grippe]

Moby Thesaurus II by Grady Ward, 1.0:

186 Moby Thesaurus words for "flu":
   African lethargy, Asiatic cholera, Asiatic flu, Chagres fever,
   German measles, Haverhill fever, Hong Kong flu,
   acute articular rheumatism, acute bronchitis, adenoiditis, ague,
   alkali disease, aluminosis, amebiasis, amebic dysentery,
   amygdalitis, anthracosilicosis, anthracosis, anthrax, asbestosis,
   asthma, atypical pneumonia, bacillary dysentery, bastard measles,
   bituminosis, black death, black fever, black lung,
   blackwater fever, breakbone fever, bronchial pneumonia,
   bronchiectasis, bronchiolitis, bronchitis, bronchopneumonia,
   brucellosis, bubonic plague, cachectic fever, catarrh,
   cerebral rheumatism, chalicosis, chicken pox, cholera,
   chronic bronchitis, cold, collapsed lung, common cold, coniosis,
   coryza, cowpox, croup, croupous pneumonia, dandy fever,
   deer fly fever, dengue, dengue fever, diphtheria, double pneumonia,
   dry pleurisy, dumdum fever, dysentery, elephantiasis, emphysema,
   empyema, encephalitis lethargica, enteric fever,
   epidemic pleurodynia, erysipelas, famine fever,
   fibrinous pneumonia, five-day fever, frambesia, glandular fever,
   grippe, hansenosis, hay fever, hepatitis, herpes, herpes simplex,
   herpes zoster, histoplasmosis, hookworm, hydrophobia,
   infantile paralysis, infectious mononucleosis,
   inflammatory rheumatism, influenza, jail fever, jungle rot,
   kala azar, kissing disease, la grippe, laryngitis, lepra, leprosy,
   leptospirosis, lipoid pneumonia, loa loa, loaiasis,
   lobar pneumonia, lockjaw, lung cancer, lung fever, madness,
   malaria, malarial fever, marsh fever, measles, meningitis,
   milzbrand, mumps, ornithosis, osteomyelitis, paratyphoid fever,
   parotitis, parrot fever, pertussis, pharyngitis, pleurisy,
   pleuritis, pneumococcal pneumonia, pneumoconiosis, pneumonia,
   pneumonic fever, pneumothorax, polio, poliomyelitis,
   polyarthritis rheumatism, ponos, psittacosis, quinsy, rabbit fever,
   rabies, rat-bite fever, relapsing fever, rheum, rheumatic fever,
   rickettsialpox, ringworm, rubella, rubeola, scarlatina,
   scarlet fever, schistosomiasis, septic sore throat, shingles,
   siderosis, silicosis, sleeping sickness, sleepy sickness, smallpox,
   snail fever, sore throat, splenic fever, spotted fever,
   strep throat, swamp fever, swine flu, tetanus, the sniffles,
   the snuffles, thrush, tinea, tonsilitis, trench fever,
   trench mouth, tuberculosis, tularemia, typhoid, typhoid fever,
   typhus, typhus fever, undulant fever, vaccinia, varicella, variola,
   venereal disease, viral dysentery, virus pneumonia, wet pleurisy,
   whooping cough, yaws, yellow fever, yellow jack, zona, zoster