Dexamethasone and other corticosteroids can mask signs of infection and impair the body's natural immune response that is important in fighting infection. Patients on corticosteroids are more susceptible to infections and can develop more serious infections than individuals not receiving corticosteroids. For example, chickenpox and measles viruses can produce serious and even fatal illnesses in patients on high doses of dexamethasone. Live virus vaccines, such as smallpox vaccine , should be avoided in patients taking high doses of dexamethasone, since even vaccine viruses may cause disease in these patients. Some infectious diseases, such as tuberculosis ( TB ) and malaria , can remain dormant in a patient for years. Dexamethasone and other corticosteroids can reactivate dormant infections. Patients with dormant tuberculosis may require treatment of the TB while undergoing corticosteroid treatment.