Whats the difference in abscess's and large fluid collection, in pelvis , are abscess's and fluid collection the one and the same ???

Diff is infection: An abscess is a fluid collection specifically due to infection. Abscesses contain pus. A fluid collection outside of the gut but within the abdominal cavity is called ascites. There are many possible causes of ascites. A cyst is a fluid collection contained within its own walls, like a water balloon. Abdominal abscesses usually are associated with fever and feeling quite sick.

Not necessarily : An Abcess is a walled off area of infection. Fluid collections can be anything, they have different characteristics on the imaging and treated differently. Some fluid collections are just watched closely, an some just go away on there own. Remember our bodies are made up of water primarily and this can, in essence, collect up anywhere.