Pathogenic Properties of Viruses

Viruses avoid the host’s immune response by growing inside cells.

Viruses gain access to host cells because they have attachment sites for receptors on the host cell.

Visible signs of viral infections are called cytopathic effects (CPE).

Some viruses cause cytocidal effects (cell death), and others cause noncytocidal effects (damage but not death).

Cytopathic effects include the stopping of mitosis, lysis, the formation of inclusion bodies, cell fusion, antigenic changes, chromosomal changes, and transformation.

Some Cytopathic Effects of Viruses

Transformed Cells in Culture

Pathogenic Properties of Fungi, Protozoa, Helminths and Algae

Fungal infections cause damage by:

• Capsules
• Toxins
• Allergic responses

Protozoan and helminths cause damage to host tissue by:

• Direct cell damage
• Metabolic waste products
• Blocking lymph flow
• Taking nutrients
• Allergic reactions

Some protozoa change their surface antigens while growing in a host so that the host’s antibodies don’t kill the protozoa.

Some algae produce neurotoxins that cause paralysis when ingested by humans.

Portals of Exit

Just as pathogens have preferred portals of entry they also have definite portals of exit.

• The respiratory tract via coughing or sneezing.
• The gastrointestinal tract via saliva or feces.
• The urogenital tract via secretion from the vagina or penis.
• Wounds
• Arthropods
• Syringes

Microbial Mechanisms of Pathogenicity - a Summary