and the very much later appearance of tumors and immuno‐pathologies is a consequence of a long incubation period between initial infection and ultimate pathology.
Classification of human disease–causing viruses according to virus–host dynamics
We can use the nature of the virus–host interaction to generate a simple classification of viruses, especially those of humans. These criteria are a useful aid for organizing detailed information concerning diseases with a viral etiology within the context of possible courses and outcomes. A number of specific examples are outlined in this chapter.
Some of the basic criteria that can be incorporated into such a scheme include:
1 Do the symptoms of the viral disease take a short or long time to develop after first encountering the virus?
2 Are the symptoms of initial infection relatively mild or severe?
3 Can the infected individual be expected to recover completely, or partially?
4 Does the virus stay associated with the victim following apparent recovery?
5 If the association is lasting, is the virus maintained in an infectious form either sporadically or constantly?
Viral diseases leading to persistence of the virus in the host are generally associated with viruses having long associations with human populations
Humans, like other animals, are subject to numerous viral infections mediated by viruses either maintained solely in the host population or in another population of animals with which humans interact. We have seen that two basic patterns occur in the course of virus replication in humans – persistent infections with incomplete virus clearing, and acute infections with efficient virus clearing upon recovery from the acute infection. It might be argued that persistent infections represent associations between virus and host that have stabilized over time, such that the viruses are maintained within the host population without a large negative effect. Conversely, acute infections may involve viruses that have recently moved from a different host. In the case of humans, such viruses may originate from zoonotic infections. While influenza A viruses and hantaviruses are examples that support this model, human rhinoviruses, among others, would not, since there are no known animal reservoirs. These virus groups and some of the symptoms caused by their infections of humans are listed in Table 4.1.
Table 4.1 Some viruses infecting humans.
| Family | Genome | Primary Reservoir | How Long Associated with Humans | Virus Type | Acute Disease | Primary Infection | Mortality Rate | Persistent/Latent? | Reactivation | Chronic Disease/Complications |
|---|---|---|---|---|---|---|---|---|---|---|
| Herpesviridae | DNA | Humans | Ancient | HSV‐1 | Facial lesion | Epidermis | Nil | Yes | Frequent at site | Encephalitis (rare) |
| HSV‐2 | Genital lesion | Epidermis | Nil | Yes | Frequent at site | Encephalitis (rare) | ||||
| VZV | Chickenpox | Epidermis | Nil | Yes | Once | Shingles/disseminated infection upon immune suppression | ||||
| HCMV | Mononucleosis | Hematopoietic tissue | Nil | Yes | Asymptomatic/infrequent? | Disseminated infection upon immune suppression/retinitis | ||||
| EBV | Mononucleosis | Lymphoid tissue | Nil | Yes | Asymptomatic/infrequent? | Lymphoma/carcinoma | ||||
| HHV‐6 | Roseola | Lymphoid tissue | Nil | Yes | Asymptomatic/infrequent? | ? | ||||
| HHV‐7 | Roseola | Lymphoid tissue | Nil | Yes | Asymptomatic/infrequent? | ? | ||||
| HHV‐8 | ? | Lymphoid tissue | Nil | Yes | Asymptomatic/infrequent? | Kaposi's sarcoma | ||||
| Polyomaviridae | DNA | Humans | Ancient | JC | None | Kidney/bladder | Nil | Yes | Infrequent (?) shedding | Encephalitis upon immune suppression |
| BK | None | Kidney/bladder | Nil | Yes | Infrequent (?) shedding | Kidney infection | ||||
| Papillomaviridae | DNA | Humans | Ancient | >60 types | Warts | Epidermis | Nil | Yes | Constant shedding at site | Cervical carcinoma (Types 6, 11, 16, and 18) |
| Adenoviridae | DNA | Humans | Ancient? | >12 types | Mild respiratory |
Respiratory tract
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