You can board with the flu, but should not: you are most contagious in the first 3 days, cabins concentrate exposure, and pressure changes can turn congested ears into barotrauma.
Flu spreads from a day before symptoms until 5 to 7 days after, and a plane is a bad place to be the source. The physical misery argument is personal: blocked sinuses plus pressure changes equal severe ear pain and sometimes eardrum injury. Airlines can also deny boarding to visibly sick passengers, and some destinations screen for fever. The playbook: rebook if you are in the acute fever-and-aches phase, since airlines often waive change fees for documented illness; if travel is unavoidable, wait until 24 hours fever-free without medication, mask the entire trip, sanitize, and hydrate. Post-flight ear problems (blocked, painful after landing) usually clear with yawning, swallowing, and gentle Valsalva; severe pain, persistence beyond a couple of days, or hearing loss warrants a visit.
What to do
Common questions
Can I fly with the flu?
Legally and physically you can board, but you should not, for three reasons. One, you are a public health hazard: flu is most contagious in the first 3 days of symptoms (and spreadable from a day before symptoms until 5 to 7 days after), and a plane cabin packs strangers shoulder to shoulder in recirculated air for hours. Two, you will suffer more: cabin pressure changes on takeoff and landing are miserable with congested ears and sinuses, and can cause genuine ear barotrauma (severe pain, temporary hearing loss, occasionally a ruptured eardrum). Three, airlines can deny boarding to visibly ill passengers, and some countries screen arriving travelers for fever. The practical guidance: if you are in the acute phase (fever, body aches, chills), rebook; most airlines waive change fees for documented illness if you call, and travel insurance often covers it. If you must travel while recovering: wait until fever-free for 24 hours without medication, mask the entire trip, sanitize hands, and hydrate aggressively. If you have already flown while sick and now your ear is blocked or painful after landing: yawning, swallowing, and the Valsalva maneuver usually clear it within hours to days; pain that is severe or lasts beyond a day or two, or any hearing loss, deserves a doctor visit.