How Viruses Work: Structure, Replication, and Disease

What Is a Virus

A virus is an infectious particle composed of nucleic acid enclosed in a protein shell called a capsid. Unlike bacteria, fungi, and other microorganisms, viruses are not cells. They lack ribosomes, metabolic enzymes, and the ability to generate their own energy. A virus particle, called a virion, is essentially a set of genetic instructions wrapped in a protective coat. It cannot grow, divide, or carry out any metabolic process on its own. Only when a virus infects a host cell can its genetic program be executed, producing new virus particles at the expense of the host.

This inability to replicate independently has fueled a long-running scientific debate about whether viruses should be considered alive. They possess some characteristics of life, including the ability to evolve through natural selection, but they lack others, such as cellular structure and independent metabolism. Most biologists regard viruses as obligate intracellular parasites that occupy a gray area between the living and the nonliving. Regardless of their classification, viruses are the most abundant biological entities on Earth, with an estimated 10^31 individual particles in the biosphere, outnumbering all cellular organisms combined.

Viral Structure

All viruses share two basic structural components: a nucleic acid genome and a protein capsid. The genome can be DNA or RNA, single-stranded or double-stranded, linear or circular. DNA viruses, such as herpesviruses and adenoviruses, tend to have larger and more stable genomes. RNA viruses, such as influenza virus and coronaviruses, typically have smaller genomes and higher mutation rates because RNA replication lacks the error-correcting mechanisms found in DNA replication. Some viruses, called retroviruses (including HIV), carry an RNA genome but use a special enzyme called reverse transcriptase to convert their RNA into DNA after infecting a host cell.

The capsid is assembled from multiple copies of one or a few types of protein subunits called capsomeres. Capsids come in several geometric forms. Icosahedral capsids have 20 triangular faces and approximate a sphere, giving the virus a roughly round appearance; many common viruses, including adenoviruses and poliovirus, have this structure. Helical capsids are rod-shaped or filamentous, with the capsid proteins arranged in a spiral around the nucleic acid; tobacco mosaic virus is the classic example. Complex capsids, found in some bacteriophages, combine elements of both icosahedral and helical symmetry and may include additional structures such as tail fibers used for attachment to host cells.

Many animal viruses possess an additional outer layer called an envelope, a lipid membrane derived from the host cell during the budding process. Embedded in this envelope are viral glycoproteins that the virus uses to recognize and attach to specific receptors on target cells. Enveloped viruses, including influenza, HIV, and SARS-CoV-2, are generally more susceptible to environmental inactivation (by soap, detergents, and alcohol-based sanitizers) than non-enveloped viruses because disrupting the lipid envelope destroys the virus's ability to infect cells.

The Viral Replication Cycle

The replication cycle of a virus follows a general sequence of steps, although the details vary depending on the type of virus and host cell. The first step is attachment (also called adsorption), in which the virus binds to specific receptor molecules on the surface of a host cell. This interaction is highly specific, which is why most viruses can only infect certain cell types, tissues, or species. The receptor for SARS-CoV-2, for example, is the ACE2 protein found on human respiratory epithelial cells.

After attachment, the virus enters the cell through various mechanisms. Enveloped viruses may fuse their membrane with the host cell membrane, releasing the capsid into the cytoplasm. Non-enveloped viruses may be taken up by endocytosis and then escape from the endosome. Once inside, the virus uncoats, releasing its nucleic acid into the cell. The freed genome then takes over the host cell's molecular machinery. DNA viruses typically replicate in the nucleus, using host enzymes (and sometimes their own) to copy their DNA and transcribe it into mRNA. RNA viruses usually replicate in the cytoplasm using their own RNA-dependent RNA polymerase.

The viral mRNA is translated by host ribosomes to produce viral proteins, including capsid proteins, enzymes, and (for enveloped viruses) glycoproteins. New viral genomes and proteins are assembled into complete virions. In the final step, new viruses are released from the cell. Lytic viruses cause the host cell to burst open, releasing hundreds or thousands of new particles. Enveloped viruses typically bud from the cell membrane, acquiring their lipid envelope in the process, sometimes without immediately killing the cell. Some viruses, called temperate or lysogenic viruses, can integrate their genetic material into the host genome and remain dormant for extended periods before reactivating to produce new virions.

Viral Evolution and Mutation

Viruses evolve rapidly, driven by high mutation rates, large population sizes, and short generation times. RNA viruses are especially prone to mutation because RNA-dependent RNA polymerases lack the proofreading ability of DNA polymerases, resulting in error rates roughly a million times higher than those of cellular DNA replication. This high mutation rate generates enormous genetic diversity within viral populations, allowing rapid adaptation to new hosts, immune responses, and antiviral drugs.

Antigenic drift refers to the gradual accumulation of mutations in genes encoding surface proteins, which can allow the virus to partially evade existing immunity. This is why influenza vaccines must be reformulated annually. Antigenic shift is a more dramatic change that occurs when two different strains of a virus co-infect the same cell and exchange genetic segments, producing a novel combination. Antigenic shift in influenza viruses has been responsible for several pandemics, including the 1918 Spanish flu and the 2009 H1N1 pandemic.

Recombination, the exchange of genetic material between co-infecting viruses, also drives viral evolution. Coronaviruses are particularly prone to recombination because of their unusually large RNA genomes and the tendency of their polymerase to switch between RNA templates during replication. This ability to recombine may have contributed to the emergence of SARS-CoV, MERS-CoV, and SARS-CoV-2 from animal reservoirs.

Major Human Viral Diseases

Viruses cause a staggering range of human diseases. Respiratory viruses, including influenza, respiratory syncytial virus (RSV), and coronaviruses, are among the most common causes of illness worldwide. HIV, a retrovirus that attacks CD4+ T cells of the immune system, has killed over 40 million people since the beginning of the AIDS epidemic and remains a major global health challenge despite advances in antiretroviral therapy. Hepatitis B and C viruses cause chronic liver infections that can lead to cirrhosis and liver cancer. Human papillomaviruses (HPV) cause cervical cancer and other malignancies. Ebola and Marburg viruses cause hemorrhagic fevers with very high mortality rates.

Antiviral drugs are available for some viral infections but are generally less effective and less broadly applicable than antibiotics are for bacterial infections. Vaccines remain the most powerful tool for preventing viral diseases. Inactivated vaccines, live attenuated vaccines, subunit vaccines, and the newer mRNA vaccines each stimulate the immune system to recognize and respond to viral antigens, providing protection against future infection. The success of the global smallpox eradication campaign, completed in 1980, demonstrated that vaccination can eliminate a viral disease entirely.

Viruses in Medicine and Biotechnology

Despite their reputation as agents of disease, viruses are increasingly being harnessed for beneficial purposes. Bacteriophages, viruses that infect bacteria, are being investigated as alternatives to antibiotics for treating drug-resistant bacterial infections, an approach known as phage therapy. Because each phage typically targets a narrow range of bacterial species, phage therapy can potentially eliminate a pathogen without disrupting the broader microbiome, a significant advantage over broad-spectrum antibiotics.

Viral vectors are widely used in gene therapy, where modified viruses deliver therapeutic genes to patient cells. Adeno-associated viruses (AAV) and lentiviruses are among the most commonly used vectors because they can efficiently deliver genes to target cells with minimal toxicity. This approach has yielded approved treatments for inherited diseases such as spinal muscular atrophy and certain forms of inherited blindness. Oncolytic viruses, engineered to selectively infect and destroy cancer cells while sparing normal tissue, represent another promising frontier in viral biotechnology.