How resilient are fetuses insude the womb to infection or general outside triggers?

In general: Fortunately for the fate of humanity fetuses are quite well protected inside the uterus. Otherwise the human race would never have survived. There are many infections that can cause problems during pregnancy. The specific risks of each would need to be assessed separately

Depends: It depends on the type of infection, and especially if it penetrates the womb - Chorioamnionitis, which put the fetus at high risk for demise. Outside influences like smoking, drugs, and alcohol, that can penetrate the placenta also adversely affect the fetus.