A Study to Evaluate the Safety and Tolerability of ABBV-0805 in Patients With Parkinson's Disease

Inclusion Criteria: * Diagnosed with idiopathic Parkinson's Disease (PD) within 5 years and with modified Hoehn and Yahr Stage of less than 3 at Screening. * Body Mass Index (BMI) is \>= 18.0 to \<=35.0 kg/m2. * Participant must follow protocol-specific methods of contraception, if applicable. * Participant must be in general good health (except for PD) based upon results of medical history, physical examination, vital signs, laboratory testing, neurological examination and 12-lead electrocardiogram (ECG). Note: If participant is taking standard of care medication for treatment of PD, doses must be stable for at least 30 days prior to starting study drug and participant should not have any clinically relevant motor fluctuations. Exclusion Criteria: * Participant with a history of, or screening brain MRI scan indicative of significant abnormality, including, but not limited to, prior hemorrhage or infarct \> 1 cm3, \> 3 lacunar infarcts, cerebral contusion, encephalomalacia, aneurysm, vascular malformation, subdural hematoma, hydrocephalus, space occupying lesion (such as an abscess or brain tumor such as meningioma). * Received any drug by injection within 30 days or within a period defined by 5 half-lives, whichever is longer, prior to study administration, unless approved by the Investigator in consultation with the AbbVie Therapeutic Area Medical Director. * Treated with any investigational product within a time frame equal to 5 half-lives, if known, or within 6 weeks (for small molecules) or 6 months (for monoclonal antibodies or other biologics) prior to the first dose of study drug. * Participant with recent history of drug or alcohol abuse (within 6 months prior to study drug administration) that could impact adherence to the protocol in the opinion of the investigator. * Participant with evidence of dysplasia or history of malignancy with the exception of excised or treated cervical cancer, some indolent malignancies (such as basal cell carcinoma or squamous cell carcinomas), remission from any malignancy for more than 5 years or participants with slow growth prostatic carcinoma may be eligible to participate with the permission of the AbbVie TA MD. * Participant with history of seizure disorder or unexplained blackouts or history of a seizure within 6 months. * Participant with congenital structural or conduction abnormalities, cardiomyopathy, myocardial infarction, cardiac arrhythmias or other cardiac conditions. * Participant with varicella or herpes zoster virus infection or any severe viral infection within 6 weeks before randomization. * Received any live vaccine within 4 weeks prior to the first dose of study drug, including but not limited to: measles/mumps/rubella vaccine, varicella zoster virus vaccine, oral polio vaccine, and nasal influenza vaccine. * Participant with symptoms of an active infection or history of prior infection (viral, fungal, or bacterial) requiring hospitalization or IV antibiotics within 8 weeks before first dose of study drug. * Participant with history of abnormal laboratory result that, in the opinion of the investigator, are indicative of any significant cardiac, endocrine, hematological, hepatic, immunologic, metabolic, urologic, pulmonary, gastrointestinal, dermatologic, psychiatric, renal, neurological, and/or other major disease. * Participant with contraindications to lumbar puncture (such as lumbar scoliosis, coagulopathy, infected skin at needle puncture site). Use of anticoagulants may be allowed in the study but must be temporarily suspended prior to and after lumbar puncture. * Participant with contraindications to MRI (such as aneurysm clip, metal fragments, internal electrical devices such as a cochlear implant, spinal cord stimulator or pacemaker), are allergic to gadolinium, or have claustrophobia. * Participant currently enrolled in another interventional clinical study. Participants enrolled in non-interventional studies may be eligible to participate at the discretion of the AbbVie TA MD. * Participant with clinically significant and/or unstable medical conditions or any other reason that the Investigator determines would interfere with participation in this study or would make the participant an unsuitable candidate to receive ABBV-0805.