Blood in the stools. Many types of E. coli do no harm to humans, while others result in a period of not very serious diarrhea. However, E. coli is quite different and can result in a serious infection. The effects usually begin around three to four days after infection. This period can actually vary between a day and a week. To diagnose such an E. coli infection, one must send a sample of the patient’s stools to a pathological laboratory so that the bacteria could be cultured and tested. Testing involves identifying strains of STEC and searching for traces of toxins.