ABSTRACT
Viruses are considered to be among the smallest living particles known to humans. Figure 3.1 in Chapter 3 illustrates the comparative sizes of different types of particles. Because of their small size, viruses are extremely difficult to detect and characterize. Since they can cause many diseases in humans and can also affect crops and domestic animals, they are of great concern to public health authorities because they can exert major economic impacts by causing plant and animal diseases.
