Virions come in a wide variety of shapes and sizes (Figure 23–11), and although most have relatively small genomes, genome size can vary considerably. The recently discovered giant viruses of amoebae, called pandoraviruses, are the largest known viruses, with 700 nm particles and double-stranded DNA genomes of over 2,000,000 nucleotide pairs. The virions of poxvirus are also large: they are 250–350 nm long and enclose a genome of double-stranded DNA of about 270,000 nucleotide pairs. At the other end of the size scale are the virions of parvovirus, which are less than 30 nm in diameter and have a single-stranded DNA genome of fewer than 5000 nucleotides.
Viral genomes are packaged in a protein coat, called a capsid, which in some viruses is further enclosed by a lipid bilayer membrane, or envelope. The capsid is made of one or several proteins, arranged in regular arrays that generally produce structures with either helical symmetry, which results in a cylindrical structure (for example, influenza, measles, and bunyavirus), or icosahedral symmetry (for example, poliovirus and herpesvirus; see Figure 23–11). Some viruses instead produce capsids with more complicated structures (for example, poxviruses). When the capsid is packaged with the viral genome, the structure is called a nucleocapsid. The nucleocapsids of nonenveloped viruses usually leave an infected cell by lysing it. For enveloped viruses, by contrast, the nucleocapsid is enclosed within a lipid bilayer membrane that the virus acquires in the process of budding from the host-cell plasma membrane, which it does without disrupting the membrane or killing the cell (Figure 23–12). Enveloped viruses can cause persistent infections that may last for years, often without noticeable deleterious effects on the host.
Because the host cell performs most of the critical steps in viral replication, the identification of effective antiviral drugs that do not harm the host can be difficult. Probably the most effective strategy for containing viral diseases is through vaccinating of potential hosts. Highly successful vaccination programs have effectively poliovirus HIV Figure 23–11 examples of viral infuenza coronavirus rabies virus mumps virus(AIDS virus) virus (common cold) morphology. As shown, both DNA and shape.
eliminated smallpox infection from the planet, and the eradication of poliomyelitis is approaching completion (Figure 23–13).
Infectious diseases are caused by pathogens, which include viruses, bacteria, and fungi, as well as protozoan and metazoan parasites. All pathogens must have mechanisms for entering their host and for evading immediate destruction by the host. The great majority of bacteria are not pathogenic to humans. Those that are pathogenic produce specific virulence factors that mediate the bacteria’s interactions with the host; these proteins change the behavior of host cells in ways that promote the replication and spread of the bacteria. Eukaryotic pathogens such as fungi and protozoan parasites typically pass through several different forms during the course of infection; the ability to switch among these forms is usually required for these pathogens to survive in a host and cause disease. In some cases, such as malaria, parasites must pass sequentially through several host species to complete their life cycles. Unlike bacteria and eukaryotic parasites, viruses have no metabolism of their own and no intrinsic ability to produce the proteins encoded by their DNA or RNA genomes; they rely on subverting the machinery of the host cell.
reported cases of polio per100,000 population