a ogenes s o ra n ec ons Iceberg concept of Infection Severe symptoms Mild symptoms Infection but no symptoms Exposure but no infection Pathogenesis of Viral Infections Interaction between viral & host factors (lead to disease production) Virus Transmission Pathogenesis Viral Tropism Viruses tend to exhibit organ & cell specificities Following viral infection, there are many different patterns of localization & dissemination Determinants of viral tropism include yPresence ofspecific cell surface receptors for that virus yPresence ofcellular protein required for viral replication yPhysiologic factors Viral Virulence Not an absolute properly of a virus Depends on yViral strainyRoute of infection yDose of virus yHost oGenetic susceptibility oImmune sufficiency oAge yPresence ofcofactor (agent that modulate interaction between virus & host) oSexually transmitted infections potentiate infection by HIV oRepeated attac ks by malarial parasite potentiate development of Burkitts lymph oma by Epstein-Barr virus Viral Replication Does not necessarily cause cell death yInterfere with normal cell functions (can be subtle) yInhibition of cellular apoptosis (prolong cell life) Does not necessarily lead to new progeny virus Abortive infections, Latent infections Abortive Viral Infections Viral Latency Incomplete viral replicative cycle yCell may be non-permissive & unable to support expression ofall viral genes yInfecting virus may be defective (lack some functional viral gene) Example abortive poliomyelitis (fever, malaise, drowsiness, headache, nausea, vomiting, constipation, sore throat, recovery in few days) Persistence of viral genome yNo production of progeny virions (virion in non-replica ting state) (no virus can be recovered between recurrences) yUnder specific stimuli, reactivate s to produce progeny virions Local stimuli Systemic stimuli Injury to tissues innervated by latently infected neurons Bacterial/ viral infections, menstruation, physical or emotional stress, hormonal imbalance Permits persistence of virus in presence of a fully developed immune responses Allow lifelong infection of host
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