Humans occasionally become infected with common colds and flu after flying on crowded airplanes. which microscopic biological hazard is most likely responsible?

The microorganism responsible for the common cold is a group of viruses known as the Influenza Viruses.

As they are viruses, it is important to remember that they cannot be treated with antibiotics. Antibiotics are useful only against bacterial infections.

The best treatment for viruses is the body's own immune system. However, for dangerous strains, vaccines are developed. These "teach" the immune system to recognize the virus in question and destroy it rapidly.