The purpose of this study is to investigate the total body biodistribution and radiation dosimetry of 11C-JNJ-63779586 by Positron Emission Tomography (PET) in healthy young adult males (Part A); to estimate and compare the uptake, distribution, and clearance of 11C-JNJ-63779586 in the brain by PET between mild Alzheimer Disease (AD) participants (males/females) and age- and gender-matched control participants (Part B), corrected for regional cerebral blood flow differences; and to model the tissue specific kinetics of 11C-JNJ-63779586 in human brain with the appropriate input function (IF) (Part B).

Inclusion Criteria:

Part A

• Healthy men between 18 and 55 years of age, inclusive
• Body mass index (BMI) between 18 and 30 kilogram per meter square (kg/m^2) ², inclusive, and a body weight of not less than 50 kilogram (kg) Part B
• Men or women with mild Alzheimer's Disease (AD), age- and gender-matched control participants, between 55 and 85 years of age, inclusive
• BMI between 18 and 35 kg/m^2, inclusive, and a body weight of not less than 50 kg
• Mild AD participants will be amyloid positive and have a mini-mental state examination (MMSE) greater than or equal to (>=) 20. The matched control participants will be amyloid negative and have a MMSE >= 26, respectively

Exclusion Criteria:

• History of or current clinically significant medical illness including (but not limited to) cardiac arrhythmias or other cardiac disease, hematologic disease, coagulation disorders (including any abnormal bleeding or blood dyscrasias), lipid abnormalities, significant pulmonary disease, including bronchospastic respiratory disease, diabetes mellitus, hepatic or renal insufficiency (estimated glomerular filtration rate [eGFR] within the screening period of less than 60 milliLitre per minute per 1.73 meter square [mL/min/1.73 m^2], thyroid disease, neurologic or psychiatric disease, infection, or any other illness that the investigator considers should exclude the participant or that could interfere with the interpretation of the study results
• Clinically significant abnormal values for hematology, clinical chemistry, or urinalysis at screening or at admission to the study center as deemed appropriate by the investigator
• Participant has a clinically relevant abnormal physical- or neurological examination, vital signs or 12-lead electrocardiogram (ECG)
• Participant has a history of epilepsy or fits or unexplained black-outs other than vasovagal collapse within 10 years before screening
• Participant has past or planned exposure to ionizing radiation that in combination with the planned administration with the study Positron Emission Tomography (PET) ligand and Computerized Tomography (CT) scan would result in a cumulative exposure that exceeds local recommended exposure limits