This study aims to understand how our immune system (the body's defense against germs) responds to the flu, and how the flu spreads. The flu virus used is called A/Texas/71/2017 (H3N2), specifically made for research. The goal is to help make better flu vaccines (shots) and medicines. Up to 200 healthy adults aged 18-49 can join. Participants will be admitted to Emory University Hospital where they may receive the flu virus as a nasal spray or through contact with infected people. They will stay in the hospital for 8-12 days to be watched. Follow-up visits will happen at the Hope Clinic of Emory Vaccine Center. Volunteers will be paid for their time. Joining the study is free, no costs involved.

• The study lasts about 5 months with an 8-12 day hospital stay.
• Participants must be healthy adults aged 18-49.
• Participants will receive compensation for their time and effort.