The hot salsa is bottled elsewhere, the mild salsa was made on site (with a shelf life of a few days). The mild salsa includes green onions. The contamination may have been limited to less than one small bunch on green onions.
The attack rate was very high. Since the salsa is served with free chips, most eat the salsa while waiting for their meals. The number of positives reported so far is 575, but I believe all or most were called in by local physicians, so there were probably many more who either developed mild symptoms or didn't go to the doctor because of insurance / convenience issues. The number infected was probably closer to 1000 than 500.
Since most infected dined at Chi Chi's during 3 days, which included a weekend, it seems likely that less than 2000 meals were served and some might prefer the hot salsa over the mild, so the attack rate may have been in excess of 50% for anyone eating mild salsa during the peak days.