Pope John Paul II writes in Evangelium Vitae that certain situations require prayer and fasting:
Jesus himself has shown us by his own example that prayer and fasting are the first and most effective weapons against the forces of evil (cf. Mt 4:1-11). As he taught his disciples, some demons cannot be driven out except in this way (cf. Mk 9:29). Let us therefore discover anew the humility and the courage to pray and fast so that power from on high will break down the walls of lies and deceit.The present would seem to be one of those times when a highly effective weapon against the forces of evil is needed. Pray without ceasing.