Coronavirus disease 2019 (COVID-19) is an emerging infectious disease caused by a dangerous virus named SARS-CoV-2. The most important symptoms are fever, cough, fatigue, and breathing problems. In the most serious forms of the disease, the appearance of an acute respiratory distress syndrome caused by the virus can be deadly, especially when people are fragile due to their age or in case of comorbidities. The exacerbated innate immune response could be another deadly complication. Different strategies of treatments are proposed for COVID-19 such as inhibition of virus entry by blocking ACE2 receptor used by COVID-19, inhibition of virus replication by using replication inhibitors, immunomodulatory agents to stimulate a strong immune response against COVID-19, and by using vaccines as an effective method for a long-term strategy for prevention of COVID-19.

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Just after Alzheimer's disease, Parkinson's disease (PD) is considered the second most prevalent chronic and progressive neurodegenerative disorder with an increased prevalence in the elderly population, affecting men slightly more frequently than women, and often resulting in disability, dependence on care, reduced quality of life, and premature death. Regarding the etiology of PD, it is a complex disease involving both genetic and lifestyle factors as well as environmental factors. In addition to the cardinal motor symptoms, non-motor symptoms and comorbidities of PD are increasingly recognized as central to the disease. These include somatic symptoms, such as pain and autonomic dysfunction (orthostatic hypotension, bladder dysfunction, and constipation), and behavioral problems, such as dementia, impulse control disorders, sleep disorders, fatigue, and psychosis. The epidemiological study of the different symptoms and comorbidities related to PD helps to understand their causes and consequently to know how to reduce the incidence and impact of these dysfunctions in the disease.

Parkinson's disease is known by tremor, rigidity, and bradykinesia. Non-motor symptoms also manifest during the disease course or even predate the appearance of motor-symptoms and that can be as troublesome as motor features. At the cellular level, the degeneration of substantia nigra neurons and the low levels of dopamine caused by intracytoplasmic inclusions of Lewy bodies that consist of α-synuclein were recognized as the putative cause of the disease and therefore physical symptoms. PD is a slow-moving illness and detection of that disease is not easy because of delayed symptoms, variation in clinical manifestations, course of progression, and biomarker profiles from person to person. Early diagnosis of PD plays a critical role in effective disease management. Here, the authors aim to give an overview of different methods of diagnosing PD with emphasis on clinical differential diagnosis that differentiate it from other parkinsonian disorders, challenges, and a brief description of new approaches of artificial intelligence technologies as a tool for better prediction of that disease.

Stem cells are defined by their ability of self-renewal and generation of differentiated functional cell types, which are derived from the embryo and various postnatal animal sources. These cells can be divided according to their developmental potential into totipotent, unipotent, multipotent, and pluripotent, which may be of embryonic, fetal, or adult origin. These stem cells are an effective way to explore human diseases and their treatment, especially neurodevelopmental disorders. Therefore, stem cells used for cell therapy in Parkinson's disease (PD) are mainly embryonic stem cells (ES Cells), fetal neural stem cells (NSCs), adult neural stem cells, mesenchymal stem cells (MSCs) because of their plasticity, and induced pluripotent stem cells (iPSCs). They are theoretically capable of forming dopaminergic neurons that may be implanted in patients with Parkinson's disease. This chapter presents the primordial role of cell therapy by stem cells in the mechanism of Parkinson's disease, also in dopamine cell replacement based on generating neurons.