Why don't they just Apparate?

[sixhoursahead <sixhoursahead@yahoo.com>]

No: HPFGUIDX 48054

I have always been confused why people that know they are about to be 
cursed do not just Apparate.  Ok, sure sometimes they might not be 
quick enough (hard to imagine or they are surprised, or they are like 
Lily and James who cannot leave a baby behind.  But I just cannot 
believe that the Death Eaters were able to murder so many wizards.  
Is it possible that there are forms of Dark Magic that can strip 
another wizard of his powers temporarily, that maybe actually block 
them from using Apparition?