The second hit occurs upon challenge with a virus.

This may occur during transport or upon arrival at the hospital.

They were to report any unusual event that might occur upon this floor.

This would normally occur upon the death of the head of the family.

But it occurred to me, upon reflection, that that was really not what they had said.

But when the physician had gone, something occurred upon the very spot which he had left.

This occurred upon her death on 3 February 1919.

The reaction will occur upon the treatment of any carbonate with an acid.

Then it occurred upon the same day that the two strangers paid their visit?

The mines occur upon either the upper or lower surface of the leaves.