Naked viruses lack an envelope membrane and are thus able to enter cells and hijack them to make more viruses. This process is known as lysis.
Naked RNA viruses have recently emerged as a new mode of viral transmission, utilizing extracellular vesicles (EVs) to exit host cells through a nonlytic pathway. This has been demonstrated for members of the Picornaviridae, Hepeviridae and Reoviridae families.
1. Viruses that Infect Humans
Viruses can cause infections that range from mild and easy to treat, like the common cold or warts, to life-threatening and long-lasting illnesses, like hepatitis B or C, HIV or influenza. They’re also responsible for many cases of severe and sometimes deadly illness, including respiratory diseases that can lead to pneumonia, hemorrhagic diseases with severe bleeding, and viral encephalitis or meningitis.
Unlike cells, viruses can’t capture or store energy, so they must depend on their host for reproduction. But viruses do have some characteristics of living organisms: They can bind to receptors on the cell surface, and they may feature spike-shaped structures called capsids.
Viruses that lack a lipid membrane and aren’t encased in a protein shell are called nonenveloped, or naked viruses. The shape of a virus’s capsid, whether it’s icosahedral (spherical), helical or complex, can also help scientists classify a virus.
2. Viruses that Infect Animals
Like their human cousins, animal viruses have a protein shell, or capsid, and genetic material made of DNA or RNA tucked inside. They may feature an envelope—a sphere of membrane that contains lipid—or they might not. Viruses that lack the envelope are called naked viruses.
Viruses that cause disease in animals are called zoonoses, and they can also make people sick. Viruses that can be transmitted from animals to people include flu, rabies, hepatitis, cholera, and bovine spongiform encephalopathy (BSE).
Most zoonoses are caused by viruses that are spread through infected animal feces or urine. But some are spread by direct contact with infected animals or their contaminated surroundings. For example, zoonotic infections from rodents and other wild animals can be spread through improperly ventilated or poorly maintained rat and pig barns, chicken coops, cattle barns, horse stables, and slaughterhouses. Viruses can also be spread by handling contaminated tools or sharing food or water supplies.
3. Viruses that Infect Plants
There are many different viruses that infect plant tissue, most of which contain double-stranded RNA genomes. Unlike some animal viruses, most plant viruses do not have an envelope and are therefore called naked or non-enveloped.
The virus particle, or virion, enters the cell through damage to the plant cuticle and cell wall. Once inside, the virus triggers the cellular processes that mediate the expression of its own genetic information.
Typically, non-enveloped viruses leave the host cell by disrupting the membrane and causing cell lysis. This is in contrast to the majority of enveloped viruses, which are usually able to escape from cells by budding.
Plant viruses are spread by direct contact between infected and naive plants, or by mechanical transmission of sap. This can occur during agricultural practices, such as abrasion with infected tools or hands, or naturally, such as the transfer of aphids or leafhoppers from one crop to another. Symptoms of viral infection are often visible on leaves, fruit, and stems as mosaic patterns, distortion, and mottling.
4. Viruses that Infect Microorganisms
Naked viruses infect microorganisms including archaea, bacteria and single-celled eukaryotes such as algae and protozoa. Many of these are the ancestors of bacteriophage, which helped spawn molecular biology and contributed to the discovery of antibiotics.
The proteins that comprise a virus can be DNA or RNA, and either can be single or double-stranded. Regardless of the type, they are unable to reproduce without a living host. Viruses are so small that they can only be seen under a microscope, and they are often spread by direct contact between infected people, or through airborne droplets such as those produced by sneezing or coughing.
Naked viruses do not need a lipid membrane to infect cells because they can trigger cell lysis that releases nucleocapsids containing the viral genome. These particles can also survive harsh environmental conditions, such as heat and dryness. For example, researchers found that norovirus, parvovirus and hepatitis A virus (HAV) remain active on stainless steel, plastic and other hard surfaces for up to 24 hours after transmission of the viruses through hand-to-hand contact or inhalation of the viruses from these surfaces.
