Viral infections

Most viral infections cause no symptoms and do not result in disease. For example, only a small percentage of individuals who become infected with Epstein-Barr virus or western equine encephalomyelitis virus ever develop disease symptoms. In contrast, most people who are infected with measles, rabies, or influenza viruses develop the disease. A wide variety of viral and host factors determine the outcome of virus infections. A small genetic variation can produce a virus with increased capacity to cause disease. Such a virus is said to have increased virulence.

Viruses can enter the body by several routes. Herpes simplex virus and poxviruses enter through the skin by direct contact with virus-containing skin lesions on infected individuals. Ebola, hepatitis B, and HIV can be contracted from infected blood products. Hypodermic needles and animal and insect bites can transmit a variety of viruses through the skin. Viruses that infect through the respiratory tract are usually transmitted by airborne droplets of mucus or saliva from infected individuals who cough or sneeze. Viruses that enter through the respiratory tract include orthomyxovirus (influenza), rhinovirus and adenovirus (common cold), and varicella-zoster virus (chicken pox). Viruses such as rotavirus, coronavirus, poliovirus, hepatitis A, and some adenoviruses enter the host through the gastrointestinal tract. Sexually transmitted viruses, such as herpes simplex, HIV, and human papilloma viruses (HPV), gain entry through the genitourinary route. Other viruses, including some adenoviruses, echoviruses, Coxsackie viruses, and herpesviruses, can infect through the eye.

Virus infections can be either localized or systemic. The path of virus spread through the body in systemic infections differs among different viruses. Following replication at the initial site of entry, many viruses are spread to their target organs by the bloodstream or the nervous system.

The particular cell type can influence on the outcome of virus infection. For example, herpes simplex virus undergoes lytic replication in skin cells around the lips but can establish a latent or dormant state in neuron cell bodies (located in ganglia) for extended periods of time. During latency, the viral genome is largely dormant in the cell nucleus until a stimulus such as a sunburn causes the reactivation of latent herpesvirus, leading to the lytic replication cycle. Once reactivated, the virus travels from the ganglia back down the nerve to cause a cold sore on the lip near the original site of infection. The herpesvirus genome does not integrate into the host cell genome.

Virus-induced illnesses can be either acute, in which the patient recovers promptly, or chronic, in which the virus remains with the host or the damage caused by the virus is irreparable. For most acute viruses, the time between infection and the onset of disease can vary from three days to three weeks. In contrast, onset of AIDS following infection with HIV takes an average of 7 to 11 years.

Several human viruses are likely to be agents of cancer, which can take decades to develop. The precise role of these viruses in human cancers is not well understood, and genetic and environmental factors are likely to contribute to these diseases. But because a number of viruses have been shown to cause tumors in animal models, it is probable that many viruses have a key role in human cancers.

Some viruses – alphaviruses and flaviviruses, for example – must be able to infect more than one species to complete their life cycles. Eastern equine encephalomyelitis virus, an alphavirus, replicates in mosquitoes and is transmitted to wild birds when the mosquitoes feed. Thus, wild birds and perhaps mammals and reptiles serve as the virus reservoir, and mosquitoes serve as vectors essential to the virus life cycle by ensuring transmission of the virus from one host to another. Horses and people are accidental hosts when they are bitten by an infected mosquito, and they do not play an important role in virus transmission.

Lichen

Lichen, living partnership of a fungus and an alga. The fungus component is called the mycobiont and is composed of intertwined, threadlike fibers called hyphae that are tightly packed into a tissuelike sheet. The fungus uses these hyphae to absorb food from its surroundings. The algal component, called the photobiont, makes its own food through photosynthesis and grows as a mass of green cells dispersed among the fungal hyphae. Lichens survive in a wide variety of environments, either forming small, circular crusts or leaflike structures attached to bark, rocks, or soil, or as hairlike structures hanging from tree branches.

The relationship between the fungus and the alga in a lichen is an example of mutualism, in which both partners benefit from the partnership. This relationship enables each to tolerate harsh conditions where neither could survive alone. In this partnership, the fungus furnishes the alga with water, prevents overexposure to sunlight, and provides simple mineral nutrients, while the photosynthesizing alga supplies food to the fungus even if no other organic material is available. In dry, barren areas where plants have a hard time growing, such as polar tundra, deserts, rocky outcrops, or high mountains, lichens are the primary photosynthesizers. Some remarkable species even grow inside porous rocks, just below the surface where some light can still reach the algal cells.

Their ability to grow in severe conditions often makes lichens pioneers in plant succession, the process in which plants colonize bare rock or soil. Lichens release acids that break down inhospitable rock, permitting soil-trapping mosses and grasses to grab hold. In areas where soil gradually accumulates, such as a forest floor, the pioneering lichens are eventually replaced by plants and trees, although other lichens may grow on these plants and trees.

About 18,000 to 20,000 species of lichens have been identified. Scientists typically classify lichens based on the fungal partner, and the lichen name is the same as the scientific name of the fungal partner. By far the most common fungi found in lichens are sac fungi, or ascomycetes, which produce reproductive spores in special cells shaped like sacs. In about two dozen tropical lichens, the fungal component is a club fungus, or basidiomycete. The club fungi, which include common mushrooms and toadstools, produce their reproductive spores in special cells that are shaped like clubs.

The algal partner of a lichen is usually composed of green algae in the form of single cells or chains of cells. Green algae contain chlorophyll, the primary light-absorbing pigment necessary for photosynthesis. This pigment is housed in saclike structures called chloroplasts, the sites of photosynthesis. In some dark-colored, gelatinous lichens, cyanobacteria (also known as blue-green algae) are the algal component. Usually a lichen contains a single species of algae, but in a few cases both a green alga and a cyanobacterium live in different parts of the same lichen.

A typical lichen has a three-layered structure. A middle layer containing algal cells entwined in fungal hyphae is sandwiched between two layers of fungal tissue. This three-layered structure is arranged into one of three basic growth forms. Crustose lichens grow as flattened crusts with the bottom layer cemented to the surface of a rock or tree bark. Foliose lichens have a leaflike appearance, with a distinct upper and lower surface exposed to the air. Foliose lichens often form large, flaky patches on tree trunks. Fruticose lichens grow in hairlike, shrubby strands on the ground or hanging from tree branches. Each strand is tubular, with the typical three-layered structure surrounding a hollow core. A common fruticose lichen is old man's beard, which hangs in wispy clumps from tree limbs and resembles moss.

Lichens lack roots, but they are usually firmly attached to the surfaces where they grow by hyphae. Lichens grow excruciatingly slowly, adding a few millimeters to their length or diameter in a year. The fastest growing varieties may add no more than 30 mm (1.2 in) to their length in a year. Lichens may have long life spans – a lichen found in West Greenland in the Arctic is more than 4,500 years old. Scientists have used the sizes of large specimens to estimate how long it has been since glaciers covered arctic and mountain areas.

During prolonged dry periods, lichens survive by retaining a small amount of water and reducing their growth and metabolic processes to the barest survival levels. After a rain, the fungal partner is able to soak up water like a sponge, absorbing two to three times its weight in water. A moist internal environment is critical for the alga, which needs water, along with carbon dioxide and sunlight, to manufacture food through photosynthesis. During dry periods when other food may be lacking, the fungus benefits by absorbing sugars and nitrogen-rich compounds produced by the alga during photosynthesis. In some cases the fungal partner is parasitic, using special fungal hyphae called haustoria to penetrate the algal cell and absorb nourishment. Since the fungal partner usually does not kill the alga, some scientists call this relationship a controlled parasitism of the alga by the fungus.

In lichen reproduction, the fungus undergoes sexual reproduction and the alga undergoes asexual reproduction, each independently of its partner. In the fungus, the hyphae contains two mating strains, called plus and minus strains because there are no anatomical distinctions between them. The nuclei from these two mating strains fuse within a special, multicellular reproductive structure on the surface of the lichen. The fused nuclei divide several times and produce spores, which are carried away by the wind and may germinate to form a new mass of hyphae. These new fungal hyphae are free of algae and typically must link with an alga in order to survive. The algal cells within the lichen reproduce through mitosis, a process in which a single cell divides into two genetically identical cells.

Lichens as a unit may also undergo asexual reproduction. The lichen may slough off small clumps of soredia, intertwined fungal hyphae with a few algal cells, which disperse to new habitats. Alternatively, the lichen may produce small, fingerlike buds on the surface called isidia, which break off and form new lichens.

Lichens are common food for insects and slugs. In the arctic tundra, reindeer and caribou rely on lichens during the winter, when no other food is available. Several species of lichens that sprout up through the snow are called reindeer mosses.

Humans rarely eat lichens except in cases where no other food is available. The Bible may chronicle one such example: Some scholars believe that the manna or bread that the ancient Israelites ate in the desert when they fled Egypt was made of lichen. In Japan, where algae as well as fungi are prized foods, certain lichens are eaten as delicacies.

Although lichens have been used in folk medicine as purported cures for many ills, from headaches and toothaches to tuberculosis, diabetes, and asthma, their use in modern medicine is recent. The discovery in the 1940s that some fungi produce potent antibiotics stimulated an extensive screening of fungi and lichens. Since then, lichen extracts have found limited use in Europe, where lichen antibiotics have been used to treat tuberculosis and some skin diseases.

Lichen extracts are also used to add color or scent. Many lichens have earthy red, brown, russet, and blue pigments, and these pigments have been used as clothing dyes since the time of ancient Greece. Native Americans use boiled lichen extracts to dye cloth and baskets. Even the famous Harris tweeds, woolen textiles from the Scottish islands, are still dyed with Scottish lichens. A number of dark, oily extracts from European and African lichens are used to add scents to soaps and perfumes.

Lichens are used as living indicators of environmental problems because of their sensitivity to atmospheric pollution. Despite their hardiness in severe conditions, many lichens are damaged by the chemicals found in polluted air and by acid rain produced from the burning of coal, oil, and gasoline, and other industrial processes. Comparisons of lichen damage from industrialized and nonindustrialized areas provide an index of air pollution. Studies spanning the past 50 years have found that the variety and number of lichens have declined in many industrialized cities in both Europe and North America. Fortunately, studies have shown that when a pollution source is shut down, lichens can make a comeback, providing hope for their long-term survival.