Does toxoplasmosis commonly cause severe brain damage?

Depends. Toxoplasmosis came back with the aids disease. It may be common in other immunosupressions. The severity depends upon the areas affected and the amount of symptomatology pts have. It signals significant immunosuppresion.
Rare in US. Toxoplasmosis is unusual but is present in pts. With HIV infections, and in other immune-suppressed situations. 30% have seizures, and other problems include focal weakness, headaches, fever, gait imbalance. Abscesses, and meningoencephalitis can occur. Meds can result in good results in 60-90% of pts.