What is the difference between effusion and infiltrate?

Effusion refers to the accumulation of fluid in a body cavity, such as the pleural space surrounding the lungs, often due to conditions like infection or heart failure. Infiltrate, on the other hand, describes the abnormal presence of substances, such as fluid, cells, or other materials, within a tissue or organ, often indicating inflammation or infection. Essentially, effusion is about fluid accumulation in a space, while infiltrate pertains to substances permeating tissue.