Select a category above to classify viruses according to shared characteristics.
Select any virus below to explore its 3D structure and biology.
Viruses consist of genetic material (DNA or RNA) enclosed in a layer of viral proteins. Some viruses exit their host cell by budding from its surface. In the process, part of the host cell membrane envelops the virus particle forming an outer layer, called the envelope. The envelope contains host and viral proteins embedded within it. Some of these proteins (colored red in the illustrations) serve to bind to the host cells. Viruses whose replication does not involve budding from the host cell surface do not have an envelope and are referred to as "naked" or "non-enveloped."
The structure of a virus is typically described based on the overall shape of the protein layer that surrounds the virus genetic material. This layer is called the capsid, or the core in enveloped viruses. The shape of the capsid or core is determined by the arrangement of many individual proteins and is typically symmetrical. The core structure is not visible in the 3D models for enveloped viruses.
A host is the organism that a virus infects and replicates in. Viruses can only replicate inside a host cell. Viral hosts include animals, plants, bacteria, fungi, and archaea. Many viruses have evolved to infect multiple kinds of hosts, while others have a more limited host range.
All viruses contain genetic material (the viral genome) that encodes one or more proteins. Viral genomes vary by the type of nucleic acid (RNA or DNA), number of strands of nucleic acid (ss, single-stranded, or ds, double-stranded), the sense, or polarity, of the strands (+, positive, or -, negative), and the structure (circular or linear). Most viral genomes consist of a single continuous sequence either with the two ends joined together (circular genome) or with the two ends not joined (linear). However, some viruses have segmented genomes, made up of multiple independent nucleic acid segments.
The mechanism by which a virus passes from one host to another depends on several factors, including which organisms the virus is able to infect, which types of cells the virus infects, and how the virus is released from an organism (for example, through sneezed droplets or bodily fluids). Some viruses can easily be passed from person to person, whereas others depend on an intermediate organism, like an arthropod (a mosquito or tick), to transmit it. An organism that serves to transmit a virus from one host to another is called a vector. A virus that is transmitted from a vertebrate animal (for example, a rodent or bat) to humans is considered a zoonotic virus.
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 animals from diseases caused by several virus infections. Human infectious diseases for which we have effective vaccines include smallpox, polio, and seasonal influenza.