A virus, at its simplest, is a package of genetic material wrapped in a protein shell. Every virus contains two essential components: a nucleic acid genome (either DNA or RNA) and a protective protein coat called a capsid. Some viruses add a third layer, a fatty membrane called an envelope, stolen from the cells they infect. That’s the core blueprint, but the details of how these pieces fit together vary enormously across the viral world.
The Genetic Material Inside
Unlike every living cell on Earth, which stores its genetic instructions as double-stranded DNA, viruses are far more creative. Viral genomes can be DNA or RNA, single-stranded or double-stranded, arranged as one continuous piece or split into several segments, and structured as either a linear strand or a circular loop. This diversity is one reason viruses are classified into seven major groups, known as the Baltimore classification.
Those seven groups cover nearly every possible arrangement of genetic material: double-stranded DNA viruses (like herpes), single-stranded DNA viruses (like parvoviruses), double-stranded RNA viruses (like rotavirus), two different categories of single-stranded RNA viruses depending on whether the RNA can be directly read by the cell’s machinery or needs to be converted first, retroviruses like HIV that carry RNA but copy it into DNA inside the host, and a seventh group of DNA viruses that also use a reverse-copying step. The type of genome a virus carries dictates how it hijacks a cell’s machinery to reproduce.
Viral genomes are remarkably small. The tiniest contain just a few genes, barely enough to code for a handful of proteins. Giant viruses like Mimivirus and Pandoravirus break this mold, with genomes ranging from 1 to 2.5 million base pairs of DNA, rivaling some bacteria. But most viruses operate with a stripped-down genetic toolkit, relying heavily on the host cell to do the work of copying and building new virus particles.
The Protein Shell: Capsid
The capsid is the protein armor surrounding the viral genome. It serves two jobs: protecting the fragile genetic material from being destroyed by enzymes in the environment, and attaching to specific receptors on a target cell to start an infection. Capsids are built from smaller protein units called capsomeres, which self-assemble into the finished structure much like interlocking tiles. Depending on the virus, capsomeres can appear spherical, cylindrical, or ring-shaped under an electron microscope.
The genome plus its capsid together form what virologists call the nucleocapsid. In simpler viruses, the nucleocapsid is essentially the entire virus particle. In more complex viruses, additional layers of protein or a membrane surround it.
Icosahedral Capsids
Many viruses build their capsids in the shape of an icosahedron, a roughly spherical structure with 20 triangular faces. This geometry is efficient: it creates a strong, enclosed shell from a relatively small number of protein building blocks. The simplest icosahedral capsids use just 60 identical protein copies. More complex versions use hundreds or even thousands. Adenoviruses, for instance, build their capsid from 1,500 protein subunits arranged into pentamers and hexamers. Herpes simplex virus uses 960. The human papillomavirus (HPV) capsid is made of 360 copies of its major capsid protein organized into 72 capsomeres on a lattice pattern.
Helical Capsids
The other common design is the helix. Here, protein subunits spiral around the genetic material like steps in a spiral staircase, forming a rod or tube shape. Tobacco mosaic virus is the classic example, with roughly 2,130 protein subunits coiled into a rigid hollow rod. Most helical capsids belong to plant viruses and certain bacteriophages (viruses that infect bacteria). In animal viruses like influenza and Ebola, the helical nucleocapsid is flexible rather than rigid and is wrapped inside an envelope.
The Envelope
Some viruses surround their capsid with an additional outer layer: a lipid membrane called the envelope. This membrane isn’t built from scratch. The virus steals it from the host cell during its exit, budding through the cell’s outer membrane or internal membranes and taking a patch of fatty bilayer with it. Influenza, HIV, coronaviruses, and Ebola are all enveloped viruses.
The envelope has a major practical consequence. Because it’s made of a fragile lipid bilayer, enveloped viruses are not stable outside the body. They’re easily destroyed by soap, disinfectants, and drying out, which is why they typically spread through direct contact or body fluids. Non-enveloped (or “naked”) viruses, with only their tough protein capsid exposed, are far more durable. They can survive on surfaces, resist drying, and spread through routes like contaminated water or food. Poliovirus, norovirus, and adenoviruses are all non-enveloped, which helps explain why they’re so persistent in the environment.
Surface Proteins and Spikes
Studded into the envelope of many viruses are glycoproteins, proteins decorated with sugar molecules, that project outward like spikes. These spikes are the virus’s tools for finding and entering a host cell. One part of the spike protein binds to a specific receptor on the cell surface, functioning like a key fitting into a lock. A different part then triggers the fusion of the viral envelope with the cell’s membrane, opening a path for the genome to slip inside.
The coronavirus spike protein is a well-known example. Its S1 region recognizes and grabs onto a receptor on human cells, while its S2 region drives the membrane fusion step. This two-part mechanism, attachment followed by fusion, is common across enveloped viruses, though the specific proteins and receptors differ. For non-enveloped viruses, surface capsid proteins handle the attachment step instead, and the genome typically enters through a pore punched into the cell membrane by capsid components.
Enzymes Packed Inside the Particle
Some viruses carry their own enzymes inside the virus particle, pre-loaded and ready to work as soon as the genome enters a new cell. This is necessary when the host cell doesn’t have the right machinery to process the virus’s particular type of genetic material.
HIV is a prime example. It packages reverse transcriptase, the enzyme that converts its RNA genome into DNA so it can be inserted into the host cell’s chromosomes. It also carries integrase, which splices that new DNA into the host genome, and a protease that processes viral proteins into their functional forms. Negative-sense RNA viruses like influenza carry their own RNA-copying enzyme because the host cell has no way to read their backward-oriented RNA without it. These built-in enzymes are often key drug targets, since blocking them can halt the virus’s replication cycle.
Complex Viruses: Bacteriophages
Not all viruses fit neatly into the icosahedral or helical categories. Bacteriophages, the viruses that infect bacteria, often have elaborate multi-part structures that combine different symmetries into a single particle. A typical tailed bacteriophage has an icosahedral head containing its DNA genome, connected through a portal structure to a tail that may be long and contractile, long and flexible, or short and stubby.
The T4 bacteriophage is the textbook example of this complexity. Its contractile tail has two protein layers and connects the head to a baseplate assembled from at least 16 different proteins. Long fibers extend from the baseplate and from the head, serving as the machinery that recognizes a bacterial cell, locks onto it, and punches the viral DNA through the bacterial cell wall. In phages with non-contractile tails, an internal “tape measure protein” determines the tail’s length by limiting how many rings of tail protein can stack up. The short-tailed phages accomplish the same infection steps with a more compact fiber and spike arrangement.
How Big Are Viruses?
Most viruses range from 20 to 400 nanometers in diameter. For perspective, a nanometer is one-billionth of a meter, and a typical human cell is roughly 10,000 to 30,000 nanometers across. The smallest known animal viruses, parvoviruses, measure about 20 nanometers. Picornaviruses (the family that includes the common cold rhinovirus and poliovirus) come in around 30 nanometers. At the other extreme, giant viruses like Mimivirus and Pandoravirus reach 500 nanometers in diameter and up to 1,000 nanometers in length, large enough to be visible under a standard light microscope.
Despite this size range spanning roughly 50-fold, every virus shares the same fundamental challenge: it cannot reproduce on its own. It must deliver its genome into a living cell and commandeer that cell’s protein-building and energy-producing systems. The capsid, envelope, spikes, and packaged enzymes are all solutions to that single problem, refined across billions of years of evolution into an extraordinary range of shapes and strategies.