A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.
There are millions types of viruses, ~ 5,000 virus species have been described in detail. Viruses are found in almost every ecosystem and are the most abundant type of biological entity. The origins of viruses in the evolutionary history of life are unclear.
While not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles, virions, which consist of the genetic material made from either DNA or RNA, a protein coat, and in some cases an outer coat of lipids. Virion is about one one-hundredth the size of a bacterium.
A tree is a plant with an elongated stem, or trunk, supporting branches and leaves in most species. Trees tend to be long-lived, some reaching several thousand years old. Trees have been in existence for 370 million years.
A variety of plant species have independently evolved a woody trunk and branches to compete for sunlight. In more general sense, tall palms, tree ferns, bananas and bamboos are also trees. The roots branch out under the ground to anchor the tree and to extract moisture and nutrients from the soil. Above ground, the branches usually bear leaves, which capture light.
Trees reduce erosion and moderate the climate. Trees and forests provide a habitat for many species of animals and plants. Some trees provide edible fruit.