When a medication is administered intravenously, its bioavailability
is 100%. However, when a medication is administered orally, its bioavailability
decreases because not all of it is absorbed. So 70% bioavailability means that, from the dose you took, 70% of it reached systemic circulation.
"Two studies have been carried out to assess the effect of food (ie, a high-fat breakfast) on the bioavailability of a SINGLE oral dose of ribavirin. An initial study by ICN Pharmaceuticals, Inc.