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Since viruses must replicate to survive, actively infected populations are the usual source of infection. Still, some viruses such as poxviruses, noroviruses, and coronaviruses have a high resistance to desiccation. In this case a contaminated object such as a desk, pen, book, contaminated clothing, or other inanimate object can be identified as the immediate reservoir. The persistence of some viruses in fecal material is also a potentially long‐lasting, essentially passive reservoir of infection. Noroviruses, which are notorious for causing outbreaks of diarrhea on cruise ships and in nursing homes, are extremely difficult to eliminate from these facilities after an outbreak. Other viruses can even persist for long periods of time in arid environments. The last documented cases of smallpox in Somalia were apparently acquired from contaminated soil. Aerosols of infectious hantavirus and canine parvovirus can be infectious for many months after secretion. Also, some viruses, especially hepatitis A virus, can be isolated from contaminated water sources for several days or even weeks after inoculation.

Figure 3.1 Some transmission routes of specific viruses from their source (reservoir) to humans. The mode of transmission (vector) is also shown.

Source: Based on Mims, C.A. and White, D.O. (1984). Viral Pathogenesis and Immunology. Boston: Blackwell Science.

Even though infectious virus can sometimes be maintained for a time in the environment in a passive state, viruses are more commonly transmitted via contact with an infectious host. The two most usual reservoirs for human disease are other humans or other animals (zoonosis). Often this transmission requires direct contact between hosts, or may involve an intermediate vector such as a mosquito or tick; modes of spread of some human viruses are illustrated in Figure 3.1, and pathogenic viruses and their reservoirs discussed in this section are listed in Table 3.1.