Answer the following question given this paragraph: 

Infectious diseases caused by viruses include the common cold, influenza, and acquired immunodeficiency syndrome (AIDS) and are among the most significant health problems in our society. Viruses are infectious agents far smaller and simpler than bacteria that are composed of a tightly packed central core of nucleic acids enclosed in a protective shell. The shell consists of layers of one or more proteins and may also have lipid or carbohydrate molecules on the surface. Because of their simplicity, viruses must invade the cells of other organisms to be able to reproduce. Viruses are visible only under an electron microscope. They come in a variety of shapes, ranging from spherical to rod shaped. The fact that they contain either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)—but never both—allows them to be divided into two major classes: DNA viruses and RNA viruses (Figure 19.17 "Viruses"). Figure 19.17 Viruses.


Q: Culprits in the common cold and aids, what infectious agents are far smaller and simpler than bacteria?

 Choices:

- poisons

- pathogens

- parasites

- viruses

A:
viruses
