Is comma to be used after flood in this example ?
"After the flood the area was hit by an epidemic in spite of the measures urgently takan."
The sources I found say that in this case the comma is optional; specifically, when using an introductory phrase, which is also a prepositional phrase and is fewer than five words, it is fine to include or exclude the comma. If the phrase were longer than five words or contained two prepositions, the comma would be necessary:
When the sunlight struck the patio, we took shelter in the cafe.
During the production of the film, the director nearly quit.
In both these sentences, the comma is required. In addition, a comma is necessary for some other introductory phrases; participial and infinitive phrases, absolute phrases, and nonessential appositive phrases all need commas.