Flu/Cold is the most common infectious disease among humans, it is a viral infectious disease, affecting the upper respiratory system. The flu or cold is extremely contagious and is caused by influenza A or B viruses. The flu is a more severe form of the cold, and both are more common during the winter and spring months. There are over 100 different types of cold viruses, and there is no definitive cure or antibiotic for the common cold, as both the flu and cold are born from viruses.