Toxoplasmosis: This is the most likely diagnosis in both immunocompetent and immunocompromised individuals presenting with symptoms suggestive of toxoplasmosis, such as flu-like symptoms, lymphadenopathy, and in severe cases, neurological symptoms. The differentiation between the two groups largely hinges on the severity and presentation of the disease, with immunocompromised individuals more likely to present with severe or disseminated disease.

Lymphoma: Particularly in immunocompromised patients, lymphoma can present with similar symptoms to toxoplasmosis, including lymphadenopathy and systemic symptoms.
Tuberculosis: In both immunocompetent and immunocompromised individuals, tuberculosis can mimic toxoplasmosis, especially when it presents with extrapulmonary symptoms such as lymphadenopathy or neurological involvement.
Cytomegalovirus (CMV) Infection: CMV can cause a mononucleosis-like syndrome in immunocompetent individuals and severe disease in immunocompromised hosts, overlapping with the presentation of toxoplasmosis.

Central Nervous System (CNS) Lymphoma: Although less common, CNS lymphoma is critical to diagnose early, especially in immunocompromised patients, as it can present similarly to cerebral toxoplasmosis with neurological symptoms and mass lesions on imaging.
Progressive Multifocal Leukoencephalopathy (PML): Caused by the JC virus, PML is a rare but often fatal disease that affects immunocompromised individuals, presenting with progressive neurological deterioration and white matter lesions on MRI.
Cryptococcal Meningitis: This fungal infection can present with similar neurological symptoms to toxoplasmosis in immunocompromised patients and is critical to diagnose due to its high mortality rate if left untreated.

Primary CNS Vasculitis: A rare condition that can present with neurological symptoms similar to those of toxoplasmosis, including stroke-like episodes and mass lesions.
Neurosyphilis: In its tertiary stage, syphilis can cause neurological symptoms that might mimic those of toxoplasmosis, especially in immunocompromised patients.
Histoplasmosis: Although more commonly associated with pulmonary symptoms, disseminated histoplasmosis can cause neurological symptoms and should be considered in the differential diagnosis of immunocompromised patients with suspected toxoplasmosis, especially those with exposure history.