A false-negative Mantoux test result occurs when
(a) had MMR or OPV vaccination within the past month,
(b) is under-nourished or recovering from a recent acute illness or
(c) is HIV-positive, has cancer, is having treatments that suppress the immune system, or sometimes disseminated TB itself.
Simultaneous administration of MMR vaccine and Mantoux test is OK. However, the HIV test should ideally be done after a month.