An example of a short-lived fission product is iodine-131, this can also be formed as an activation product by the neutron activation of tellurium.
In both bomb fallout and a release from a power reactor accident, the short-lived isotopes cause the dose rate on day one to be much higher than that which will be experienced at the same site many days later.  This holds true even if no attempts at decontamination are made.  In the graphs below, the total gamma dose rate and the share of the dose due to each main isotope released by the Chernobyl accident are shown.