A health care provider should be consulted for a diarrheal illness is accompanied by bulleted list item, level 1 high fever (temperature over 101.5 F, measured orally) bulleted list item, level 1 blood in the stools bulleted list item, level 1 prolonged vomiting that prevents keeping liquids down (which can lead to dehydration) bulleted list item, level 1 signs of dehydration, including a decrease in urination, a dry mouth and throat, and feeling dizzy when standing up. bulleted list item, level 1 diarrheal illness that lasts more than 3 days Do not be surprised if your doctor does not prescribe an antibiotic. Many diarrheal illnesses are caused by viruses and will improve in 2 or 3 days without antibiotic therapy. In fact, antibiotics have no effect on viruses, and using an antibiotic to treat a viral infection could cause more harm than good It is often not necessary to take an antibiotic even in the case of a mild bacterial infection. Other treatments can help the symptoms, and careful handwashing can prevent the spread of infection to other people. Overuse of antibiotics is the principal reason many bacteria are becoming resistant. Resistant bacteria are no longer killed by the antibiotic. This means that it is important to use antibiotics only when they are really needed. Partial treatment can also cause bacteria to become resistant. If an antibiotic is prescribed, it is important to take all of the medication as prescribed, and not stop early just because the symptoms seem to be improving.