Petri dish for Keflex antibiotic susceptibility testing. Please note: antibiotics are absolutely ineffective for viral diseases. Viruses are many times smaller than bacteria, and antibiotics simply do not have a point of application of their ability. Therefore, antibiotics for colds have no effect, since colds in 99% of cases are caused by viruses. Antibiotics for coughs and Keflex can be effective if bacteria are causing the symptoms. Only a doctor can figure out what caused the disease - for this he prescribes blood tests, if necessary, a sputum test, if it leaves.