viruses
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 bothallows them to be divided into two major classes: DNA viruses and RNA viruses Figure 19.17 "Viruses" . Figure 19.17 Viruses.