Viruses are segments of nucleic acid wrapped in a protein “coat.” Their genetic material may be either RNA or DNA, with the coding portions devoted primarily to the instructions needed for replication. The viral genome may contain just a few genes or several hundred genes, usually arranged into a single, straight or circular structure.

All viruses are parasites – they depend on host cells for the molecular components they need to create copies of their genomes and protein coats. When a virus enters a host, it hijacks the cell’s functions for its own purposes. Under unfavorable conditions, viruses can remain dormant for long periods.

Virus Structure
Viruses exhibit a large range of sizes and shapes. They may be simple spheres or rods, or take more intricate forms such as spirals or tailed polyhedrons. Many bear external projections to help them identify and invade their hosts.

Regardless of shape, all viruses outside a host cell have two basic parts – a gene sequence made up of RNA or DNA, and a protein covering called a capsid. Some viruses also have a protective envelope made of lipids snatched from the cell membrane of their host. Others have an elongated tail attached to the capsid.

Once a virus enters a cell, it sheds its protective capsid and exists as just a length of nucleic acid.

Infection
The capsid of a virus includes proteins that are able to recognize a suitable host cell when they come in contact with one. Most viruses are very host specific – they attach only to certain types of cells, sometimes only to cells in a particular part of their life cycle. Some are more generalized, and can infect more than one species.

When a host is found, the virus attaches to the outside of the cell, and either stimulates the cell to engulf the entire virus or injects its genetic material through the cell membrane.

Replication
Viruses do not reproduce through meiosis or mitosis like cellular organisms. Instead they force their hosts to replicate them many times over. There are two interconnected types of replication, the lytic cycle and the lysogenic cycle.

During the lytic cycle, the viral genome seizes control of the host cell’s functions. It inactivates and sometimes destroys the host DNA, and directs the cell to synthesize many copies of the viral genes and proteins. These components assemble into new viruses and then burst or bud out of the cell.

During the lysogenic cycle, the viral genome insinuates itself into the host’s DNA, where it is replicated during every cell division. The viral genes can lay quiet for days, months, or years inside a cell and its descendants. But when the proper environmental trigger or genetic signal is detected, the virus breaks free of the host DNA and begins the lytic cycle.

Viral Diseases
Every living organism is susceptible to viral infection and disease. Viruses sicken and sometimes kill their hosts by destroying cells, or by preventing cells from behaving normally. Because viruses are durable, tiny, and extremely numerous, they are easily spread. Once inside their host cells, they are difficult to reach with drugs or other therapies.

Viruses are both a scourge and a boon to human health. They kill some of the other microbes that plague us, and are used in the creation of vaccines and to deliver gene therapy. They also cause innumerable illnesses, chronic conditions, and fatal diseases from colds and chicken pox to herpes, hepatitis, and cancer.

Of particular concern are viral epidemics and pandemics, which are sudden and widespread outbreaks of virulent, contagious diseases. They often occur when populations encounter novel viral strains, as when people from different areas of the world suddenly interact during war, colonization, or migration.

Outbreaks of smallpox, influenza, AIDS, and ebola have killed millions of people and their viral agents continue to defy efforts to contain or eradicate them.