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Lecture 13

BIOB51H3 Lecture Notes - Lecture 13: Sexually Transmitted Infection, Viral Load, Subtypes Of Hiv

Biological Sciences
Course Code
Maydianne Andrade

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Why is HIV Fatal?
o Natural selection within a host (rapid replication)
o Natural selection for viral transmission between hosts
o Humans have not evolved resistance to HIV
o Transmission rate hypothesis: natural selection favours increased virulence of
sexually transmitted diseases when transmission to new hosts is frequent (and
favours decreased virulence when transmission to new hosts is infrequent)
Virulence: tendency to cause disease in a host
High virulence rapid growth rate of virus
Leads to severe host illness and/or death
Low virulence slow growth rate of virus
Leads to slow development of illness or little effect on host
Evidence for the transmission rate hypothesis
Strains that kill hosts more slowly are less common globally (HIV-
o It is seldomly transmitted
Increased virulence of HIV-1 when migrants move to cities
o More opportunities for sexual partners/needle
use/hospital transmission
Directly transmitted diseases
o Requires contact between infected person and new host
Vector borne diseases
o Can be transmitted even when hosts are incapacitated
Transmitted by insect vectors or water
o Why has HIV evolved such a high degree of virulence?
Within the body of the host, HIV evolves to be more virulent over time
Reproductive Trade-offs In HIV
o High virulence has many virions/ml of blood which leads to an increased chance of
transmission at each copulation benefits the virus
o High virulence causes rapid illness and death of host which leads to a decreased number
of copulations before the death of host costly for the virus
o Evidence of reproductive trade-off for HIV
o AIDS occurs more rapidly if viral load is high
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