Cesium doesn't linger in mammals. Depending on the tissues it lodges in after inhalation or ingestion (bone, fat, muscle etc.) its biological halflife is between 70 and 120 days i.e. half the cesium taken in will be pissed away or excreted in that time, then half the residue over the next period and so on. It's the same with strontium and a number of other problem specimens in the radiochemical zoo although the half-life varies from element to element.
Iodine-131 is the major contamination problem from fission releases, it's preferentially concentrated in the thyroid and is very radioactive but because of that it goes away quite quickly, with a halflife of only 8 days or so and superdosing with iodine tablets will prevent uptake of I-131 to a large extent. Hospitals and therapeutic facilities that use I-131 to "burn out" thyroid cancers flush residues into the sewer systems leading to the occasional panic when I-131 is detected in miniscule amounts in rivers, lakes etc. downstream.