A virus is one of the simplest structures in biology, yet everything inside it is precisely engineered for one job: delivering genetic instructions into a host cell. A complete virus particle, called a virion, contains as few as two core components (a strand of genetic material and a protective protein shell) or as many as four or five distinct layers depending on the type of virus. Here’s what each of those layers is and what it does.
The Genetic Core
At the very center of every virus is its genome: a strand of either DNA or RNA that carries the instructions for making new copies of the virus. This is the entire point of the particle. Everything else exists to protect this genetic cargo and get it inside a cell where it can be read and copied.
Viral genomes come in a surprising variety of forms. Some are double-stranded DNA, similar to what’s in your own cells. Others are single-stranded DNA, double-stranded RNA, or single-stranded RNA. Some are a single continuous molecule; others are broken into multiple segments. Single-stranded viruses tend to mutate faster than double-stranded ones, and smaller genomes mutate faster than larger ones. This diversity in genetic makeup is one reason viruses are classified into seven major groups based on how their genome gets converted into usable instructions once inside a cell.
The genome doesn’t float freely inside the virus. It’s tightly wound around or threaded through specialized proteins that help compact it into an incredibly small space. The coronavirus genome, for example, is roughly 29,000 genetic letters long, yet it gets supercoiled and packed into a sphere less than 100 nanometers across. These genome-protein complexes, called nucleocapsids, can be helical (coiled like a spring) or roughly spherical, with the protein forming a core about 9 to 16 nanometers in diameter.
The Protein Shell
Surrounding the genetic core is the capsid, a shell built entirely from protein subunits. The capsid has two jobs: it shields the fragile genetic material from enzymes that would destroy it, and it latches onto specific receptors on the surface of a target cell to begin infection.
Capsids come in two main shapes. The most common is icosahedral, a geometric form with 20 triangular faces that looks roughly like a soccer ball. This design is efficient because it creates a strong, symmetrical container from many copies of just one or a few different proteins. The individual protein clusters visible on the surface are called capsomeres, and they typically group into units of five (pentons) or six (hexons) that tile together across the icosahedron’s faces. Viruses can scale up the size of an icosahedral capsid by adding more protein subunits to each triangular face.
The second shape is helical. In these viruses, the protein subunits wind around the nucleic acid in a spiral, creating a long tube or rod. Tobacco mosaic virus is the classic example. A smaller number of viruses, like certain bacteriophages (viruses that infect bacteria), have complex structures that don’t fit neatly into either category.
The Lipid Envelope
Many viruses, including influenza, HIV, and coronaviruses, have an additional outer layer called an envelope. This is a membrane made of lipids (fats) stolen directly from the host cell during the virus’s exit. The most common source is the cell’s outer membrane, though some viruses grab their envelope from internal cell structures like the endoplasmic reticulum or the Golgi complex.
The envelope isn’t just a passive wrapper. Studded across its surface are virus-encoded glycoproteins, specialized molecules that stick out like spikes. These glycoproteins handle critical tasks: recognizing and binding to receptors on the next target cell, fusing with that cell’s membrane to get inside, and helping new virus particles bud out of an infected cell. The spike protein on SARS-CoV-2 is one well-known example. Because the envelope is a stolen piece of cell membrane, it can also carry some of the host cell’s own proteins, which may help the virus avoid immediate detection by the immune system.
One practical consequence of having a lipid envelope: these viruses are relatively fragile outside the body. Soap and alcohol dissolve lipid membranes easily, which is why handwashing is so effective against enveloped viruses. Non-enveloped viruses, protected only by their tough protein capsid, tend to survive longer on surfaces.
Matrix Proteins
In enveloped viruses, there’s often a structural layer between the capsid and the envelope called the matrix. In influenza, for instance, a scaffold of matrix proteins (called M1) lines the inner face of the lipid envelope, connecting it to the viral genetic material packed inside. This layer acts as a bridge, maintaining the virus’s shape and coordinating the assembly of new particles. Without it, the envelope and the internal components wouldn’t hold together as a functional unit.
Enzymes Packed for Immediate Use
Some viruses carry their own enzymes inside the particle, tools they need the moment they enter a host cell because the cell doesn’t naturally provide them. HIV is a prime example. Inside each HIV particle are two key enzymes: one that converts the virus’s RNA genome into DNA (so it can be read by the cell’s machinery), and another that splices that newly made DNA into the host cell’s own chromosomes. These enzymes start out as part of a larger protein that gets cut into its functional pieces by a third enzyme, a protease, during the virus’s maturation.
Influenza viruses carry a different set of enzymes for copying their segmented RNA genome. Negative-sense RNA viruses (those whose genome is the mirror image of usable genetic code) must pack an enzyme that transcribes their RNA into a readable form, because the host cell has no way to do this on its own. In each case, the enzymes are loaded into the particle during assembly so they’re ready to work immediately upon infection.
The Tegument in Herpesviruses
Herpesviruses have an extra layer that most other viruses lack: the tegument, a thick protein coat between the capsid and the envelope. This layer contains anywhere from 17 to 38 different proteins depending on the specific herpesvirus. In herpes simplex virus type 1, about 24 distinct proteins occupy this space. The tegument is split into two zones. The inner tegument sits close to the capsid and mirrors its symmetrical structure. The outer tegument is more loosely organized and interacts with the envelope proteins above it.
These tegument proteins aren’t just structural padding. Many of them are released into the host cell immediately upon infection, where they begin hijacking cell processes and suppressing immune responses before the viral genome has even been unpacked. This gives the virus a head start.
How Small All of This Really Is
Everything described above fits into a particle typically between 20 and 300 nanometers in diameter. For perspective, a nanometer is one billionth of a meter. A human red blood cell is about 7,000 nanometers across, meaning you could line up dozens of smaller viruses across a single blood cell. The poliovirus is near the small end at around 30 nanometers. Larger viruses like poxviruses can reach 300 nanometers or more. Despite these vanishingly small dimensions, each particle is a precisely organized package with layers, symmetry, and functional components arranged to maximize the chance of a successful infection.