Microbiology, 2021

15.3 • Virulence Factors of Bacterial and Viral Pathogens 629
host through a portal of exit (Figure 15.9). As with portals of entry, many pathogens are adapted to use a
particular portal of exit. Similar to portals of entry, the most common portals of exit include the skin and the
respiratory, urogenital, and gastrointestinal tracts. Coughing and sneezing can expel pathogens from the
respiratory tract. A single sneeze can send thousands of virus particles into the air. Secretions and excretions
can transport pathogens out of other portals of exit. Feces, urine, semen, vaginal secretions, tears, sweat, and
shed skin cells can all serve as vehicles for a pathogen to leave the body. Pathogens that rely on insect vectors
for transmission exit the body in the blood extracted by a biting insect. Similarly, some pathogens exit the
body in blood extracted by needles.
Figure 15.9
Pathogens leave the body of an infected host through various portals of exit to infect new hosts.
15.3 Virulence Factors of Bacterial and Viral Pathogens
Learning Objectives
By the end of this section, you will be able to:
• Explain how virulence factors contribute to signs and symptoms of infectious disease
• Differentiate between endotoxins and exotoxins
• Describe and differentiate between various types of exotoxins
• Describe the mechanisms viruses use for adhesion and antigenic variation
In the previous section, we explained that some pathogens are more virulent than others. This is due to the
unique virulence factors produced by individual pathogens, which determine the extent and severity of
disease they may cause. A pathogen’s virulence factors are encoded by genes that can be identified using
molecular Koch’s postulates. When genes encoding virulence factors are inactivated, virulence in the pathogen
is diminished. In this section, we examine various types and specific examples of virulence factors and how