Cesium (symbol: Cs) is a chemical element with the atomic number 55. The human body tends to confound cesium isotopes Cs-134 and Cs-137 with Potassium, thus the radioactive get embedded in the body instead of the proper substance.
Due to above-ground nuclear weapons tests Cesium can be found in small quantities worldwide. In science external and internal exposition are distinguished. External exposition for instance takes place if you walk on contaminated ground or get in touch with radioactive material, whereas internal exposition means to incorporate radioactive substances mainly with the air (breathing) or food and drinks (ingestion). 
It has a half-life of about 30.2 years. The major part decays to metastable barium-137m while it releases beta radiation. This Ba-137m has just a half-life of 2.55 minutes and releases a strong gamma radiation which is the main indicator for Cs-137 detection with Geiger counters.
Incorporated cesium will be treated by the human organism like potassium and thus be spread throughout the whole body, particularly to the soft tissue, and partly to the heart. Due to the water solubility of most cesium salts they get completely absorbed by the gastrointestinal tract. The highest concentrations can be found in the muscles, a bit less is accumulated in bones and fat tissues. The biological half-life is 40-200 days. Internal exposition can lead to myocardium dysfunction or cancer, for instance in lungs or rectum.
Faced with high external exposition can cause burns, depending on the dose the exposition can be lethal.
After the Chernobyl disaster in 1986 Cs-134 had a significant contribution to the detected radiation doses during the first two decades. These days Chernobyl related Cs-134 is detectable only in small amounts due to its short half-life.
- https://en.wikipedia.org/w/index.php?title=Caesium&oldid=766310621 as at February 21, 2017
- ippnw factsheet: Radioactive Stoffe machen krank. A-Z von radioaktiven Isotopen, die beim Atomunfall freigesetzt werden; March 2011