The purpose of this study is to evaluate the Pharmacokinetic parameters of JNJ-63623872 in combination with oseltamivir in elderly participants (aged 65 to \<= 85 years) compared to adults (aged 18 to \<= 64 years) with influenza A infection.
Inclusion Criteria: * Participant requires hospitalization to treat influenza infection and/or to treat complications of influenza infection * Participant tested positive for influenza A infection within 1 day of signing of the informed consent form (ICF)/assent form using a polymerase chain reaction (PCR)-based rapid molecular diagnostic assay * Participants must be capable of swallowing study medication tablets and capsules * Each participant (or their legally acceptable representative) must sign an ICF indicating that he or she understands the purpose of and procedures required for the study and is willing to participate in the study * Participant must be willing and able to adhere to the prohibitions and restrictions specified in the protocol Exclusion Criteria: * Participant received more than 3 doses of the influenza antiviral medication oseltamivir, zanamivir, or peramivir since the start of the influenza symptoms, or ribavirin within 6 months prior to Screening * Participant is unwilling to undergo regular nasal Mid-turbinate (MT) swabs or has any physical abnormality which limits the ability to collect regular nasal specimens * Participant is immunocompromised, whether due to underlying medical condition (example, malignancy) or medical therapy (example, medications, chemotherapy, radiation, post-transplant) * Participant is undergoing peritoneal dialysis, hemodialysis, or hemofiltration * Participant has an estimated glomerular filtration rate (eGFR) less than or equal to (\<=)30 milliliter (mL)/minute (min)/1.73 meter\^2 (m\^2) according to the Modification of Diet in Renal Disease (MDRD) equation, assessed at Screening or based on the most recent clinically relevant creatinine value if available