Viruses are at the heart of this century-old debate. Viruses<span> are parasites that infect plants, animals, fungi and bacteria. They consist of genetic material surrounded by a protein coat and are unable to grow or reproduce without a host. The classification of viruses has confused scientists for nearly 100 years. Originally they were viewed as poisons. In fact, the word ''virus'' is Latin for ''poison.'' They have since been viewed as living creatures, then mere bio-chemical compounds, and, most recently, somewhere in between chemical compounds and lifeforms.</span>Non-cellular life<span> refers to organisms, such as viruses, that exist without any cells, and it is a very controversial topic. The cell theory, which is one of the fundamental tenets of biology, states that all living things are composed of cells and that cells are the basic units of life. So the idea that something can be considered alive without having cells is an issue that is up for debate.
<span>The second statement is correct. Viruses can infect all cellular life forms, but each cellular species
can be infected by own limited specific range of viruses. Most viruses that harm
animals can not harm humans. But, some viruses can infect different species of mammals, for example both
bats and humans can be infected be rabies.</span>
The letters of DNA pair up because they form hydrogen bonds: each contains hydrogen atoms, which are attracted to nitrogen or oxygen atoms in their partner
