An open-label, dose-ranging influenza challenge study in healthy adult volunteers to determine the optimal infection dose and safety of a recombinant H3N2 (A/Texas/ A/Texas/71/2017 (H3N2, clade 3C3a) influenza strain. The goal of this study is to find a challenge virus dose that is safe and can achieve a symptomatic influenza Attack Rate (AR) that will be sufficiently high for utilization in future vaccine or intervention studies. The optimal dose of the three considered is broadly defined as the minimum challenge virus dose that elicits the highest AR without meeting safety-stopping criteria. Additionally, viral recovery, clinical symptoms, and immune responses over the post-challenge period will be described by challenge dose group. This study will last for up to 1 year depending upon the number of challenge cohorts enrolled given the adaptive dose-escalation design. The populations are healthy males and non-pregnant, non-breastfeeding females aged = 18 and \< 46 years of age with a serum HAI antibody titer of \</=1:40 against influenza A/Texas/71/2017 (H3N2), clade 3C3a. The study will enroll and challenge up to 106 (plus 8 shams) healthy adult volunteers with the H3N2 (A/Texas/71/2017 (H3N2), clade 3C3a) influenza virus challenge strain. Subjects will be pre-screened for study inclusion to have serological HAI antibody titers of \</=1:40 against the clinical challenge strain. Eligible participants will be enrolled sequentially into dosing cohorts and will be randomly assigned to receive a single dose of either placebo (sham inoculum) or a virus dose between 104 to 106 Median Issue Culture Infectious Dose (TCID50) (in an allocation of 1:12 to 1:17). Dose titration will be conducted under an adaptive escalation schedule whereby dosing will start at the lowest dose 104 TCID50 and only escalate to the next dose if a pre-determined infection and symptomatic attack rate are not met and the dose is determined to be safe and no pre-defined halting rule is met. The attack rate (AR), defined as the percentage of subjects meeting shedding and symptom criteria for symptomatic influenza virus infection (see clinical case definition below), will be determined for each challenge dose group in order to identify the optimal infectious dose (55%-80% AR). This adaptive, dose-ranging approach allows adjustments to challenge dose group size and escalation dose schedule to be informed by the AR and safety results. The primary objectives are: 1) To determine the optimal infectious dose1 of a recombinant influenza virus (A/Texas/71/2017 (H3N2), clade 3C3a) to be used as a clinical challenge strain in future vaccine efficacy or intervention studies as assessed by viral shedding and clinical symptoms. 2) To describe viral detection by quantitative and qualitative Reverse Transcription - Polymerase Chain Reaction (RT-PCR) from study subjects at baseline and post-challenge. 3) To document clinical symptoms from self-reported surveys and standardized symptom scales at baseline and post-challenge. The secondary objectives are: 1) To assess the safety profile of a live recombinant influenza strain (A/Texas/71/2017 (H3N2), clade 3C3a) following challenge in healthy adult volunteers. 2) To describe the host serum hemagglutination inhibition and microneutralization antibody responses at baseline and post-challenge. 3) To describe anti-Hemagglutination(HA)-stalk antibody titer at baseline and post-challenge.