Virus Explorer

Viruses consist of genetic material (DNA or RNA) surrounded by a layer of viral proteins, called a capsid. Some viruses exit their host cell by budding from the cell’s surface or other membranes, which is shown in the illustration below. In the process, part of the host cell membrane envelops the virus, forming an outer layer called the envelope. Viruses with an envelope are called “enveloped.”

The envelope contains viral proteins embedded in it. Viral proteins with sugars attached to them are called glycoproteins. Glycoproteins typically bind to receptors on the membrane of a host cell.

Viruses that do not bud from the host cell membrane do not have an envelope, so they are called “naked” or “nonenveloped.” These viruses have only the capsid surrounding their genetic material.

A host is an organism that a virus infects and replicates in. Viruses can replicate only inside a host’s cell. Hosts of viruses include animals, plants, bacteria, fungi, and archaea. Many viruses have evolved to infect multiple kinds of hosts, while some viruses have a more limited host range.

All viruses contain genetic material, called the viral genome, that encodes one or more proteins. Viral genomes vary in their type of nucleic acid (RNA or DNA) and their number of nucleic acid strands (written as ss for single-stranded and ds for double-stranded).

In addition, RNA genomes vary in the sense of their strands, which is the direction in which a strand is “read,” or translated into protein. RNA genomes have either positive-sense RNA (written as (+)RNA), which can be translated directly into protein, or negative-sense RNA (written as (–)RNA), which needs to be transcribed into a complementary (+)RNA strand first.

Both DNA and RNA viral genomes also vary in structure. Most consist of a single continuous sequence, either with the two ends joined (circular) or not joined (linear). However, some viral genomes are segmented, or made up of multiple independent nucleic acid segments.

Transmission is the passing of a virus from one host to another. The mechanism by which a virus is transmitted depends on several things, including which organisms the virus is able to infect, which types of cells in those organisms the virus can infect, and how the virus is released from an organism (for example, through sneeze droplets or bodily fluids).

Some viruses are transmitted between similar types of hosts, such as from humans to humans, plants to plants, or bacteria to bacteria. Other viruses are transmitted between different types of hosts. A virus that is transmitted from an animal (for example, a rodent or bat) to humans is called a zoonotic virus.

Some viruses need an intermediate organism, called a vector, to be transmitted from one type of host to another. Arthropods, such as mosquitoes and ticks, are often vectors for transmitting certain viruses to humans.

A vaccine is a substance that, when taken into the body, should induce a protective immune response to a virus. When an individual who has been vaccinated against a virus comes into contact with that virus, the body should already be prepared to fight the infection. Scientists have developed vaccines that protect humans and some domestic animals from diseases caused by certain viral infections.