In single-particle coherent x-ray diffraction imaging experiments, performed at 
x-ray free-electron lasers (XFELs), samples are exposed to intense x-ray pulses 
to obtain single-shot diffraction patterns. The high intensity induces 
electronic dynamics on the femtosecond time scale in the system, which can 
reduce the contrast of the obtained diffraction patterns and adds an isotropic 
background. We quantify the degradation of the diffraction pattern from 
ultrafast electronic damage by performing simulations on a biological sample 
exposed to x-ray pulses with different parameters. We find that the contrast is 
substantially reduced and the background is considerably strong only if almost 
all electrons are removed from their parent atoms. This happens at fluences of 
at least one order of magnitude larger than provided at currently available XFEL 
sources.