How Viruses Work: Infection, Replication, and Why They're Hard to Kill

Not Quite Alive, Not Quite Dead

Viruses occupy a strange biological borderland. They carry genetic information — DNA or RNA — and they evolve. But outside a host cell, they are inert particles, no more alive than a crystal. They have no metabolism, cannot move, cannot grow, cannot reproduce independently. Only inside the living cell of a host organism do they spring into activity, co-opting cellular machinery to produce thousands of copies of themselves. This obligate intracellular parasitism defines virology: viruses exist entirely to replicate, and they can only do so by exploiting the molecular machinery of cells that spent billions of years evolving for entirely different purposes.

Viruses are the most abundant biological entities on Earth, outnumbering cellular life by orders of magnitude. A single liter of ocean water contains roughly ten billion virus particles. The human gut virome — the collective population of viruses in the gastrointestinal tract — contains trillions of viral particles. Most of these viruses infect bacteria (bacteriophages) and are harmless to humans. But a subset of viruses cause some of the most significant diseases in human history: smallpox, influenza, HIV/AIDS, Ebola, COVID-19. Understanding how viruses work is fundamental to understanding these diseases and to developing vaccines and antivirals to combat them.

Viral Structure: Deceptively Simple

A virus particle (virion) consists of two or three components. At the core is the genome: nucleic acid (DNA or RNA, single-stranded or double-stranded) that encodes the information needed to direct replication. Surrounding the genome is the capsid, a protein shell assembled from multiple copies of one or a few structural proteins that protect the genetic material and facilitate host cell recognition and entry. Many viruses also possess a lipid envelope derived from the host cell membrane, studded with viral glycoproteins that mediate binding to host cell receptors.

Despite this simplicity, the diversity of viral genomes and structures is vast. The Baltimore classification system groups viruses into seven classes based on genome type and replication strategy: double-stranded DNA viruses (like herpesviruses), single-stranded DNA viruses (like parvoviruses), double-stranded RNA viruses (like reoviruses), positive-sense single-stranded RNA viruses (like coronaviruses and poliovirus), negative-sense single-stranded RNA viruses (like influenza and Ebola), retroviruses (like HIV), and hepadnaviruses (like hepatitis B). Each genome type requires a different set of enzymes for replication, which has important implications for antiviral drug development.

Attachment and Entry: Getting Inside the Cell

Viral infection begins when a virion encounters and attaches to a susceptible host cell. This attachment is highly specific: viral surface proteins bind to particular receptor molecules on the cell surface with high affinity. This specificity determines host range (which species a virus can infect) and tissue tropism (which tissues within a host are susceptible). The SARS-CoV-2 spike protein binds the ACE2 receptor, which is expressed at high levels in the lungs, gut, heart, and kidneys — explaining these organs' particular vulnerability in COVID-19. The HIV gp120 protein binds CD4 and a co-receptor (CCR5 or CXCR4) expressed primarily on T helper cells and macrophages, explaining HIV's destruction of these critical immune cells.

Following attachment, viruses enter the cell through various mechanisms. Enveloped viruses fuse their envelope with the cell membrane (either at the cell surface or after endocytosis), releasing the capsid into the cytoplasm. Non-enveloped viruses typically enter through endocytosis and escape from the endosome by disrupting its membrane. Some bacteriophages inject their DNA directly through the bacterial cell wall, leaving the capsid outside. Once inside, the viral genome is released from the capsid and begins the replication program.

Genome Replication and Gene Expression

Once inside the cell, the viral genome is replicated and its genes are expressed to produce the proteins needed for new virion assembly. The details depend critically on genome type. DNA viruses generally replicate in the nucleus, using host DNA polymerase (for nuclear DNA viruses like herpesviruses) or encoding their own DNA polymerase (poxviruses, which replicate in the cytoplasm). RNA viruses face a different challenge: cells do not normally replicate RNA from an RNA template, so RNA viruses must either carry or encode their own RNA-dependent RNA polymerase (RdRp) to copy their genomes. The RdRp is essential for RNA virus replication and is a target for several antiviral drugs, including remdesivir (which targets the SARS-CoV-2 RdRp).

Retroviruses (including HIV) add an additional step: they carry an enzyme called reverse transcriptase that transcribes the viral RNA genome into DNA, which is then integrated into the host cell's chromosomal DNA by another viral enzyme, integrase. This integrated DNA — the provirus — can remain latent for years, replicated as part of the host chromosome every time the cell divides. When activated, it is transcribed and translated to produce new viral proteins and genomic RNA for assembly. This integration strategy makes HIV extraordinarily difficult to eliminate: even with antiretroviral therapy suppressing active replication, latently infected cells persist indefinitely.

Assembly and Release: The New Generation

After genome replication and gene expression, thousands of viral components accumulate in the infected cell. New virions are assembled — a process that varies widely in complexity and location. Simple viruses may self-assemble spontaneously when their components are present at sufficient concentration. Complex viruses like herpesviruses require dedicated scaffolding proteins and molecular chaperones to direct capsid assembly. The genome is packaged into the capsid through specific packaging signals — sequences in the viral genome that recruit packaging machinery.

New virions exit the host cell by lysis — rupturing the cell — or by budding. Lysis is common for non-enveloped viruses and kills the cell immediately, releasing all progeny virions simultaneously. Enveloped viruses typically bud through the cell membrane, wrapping themselves in a patch of host lipid bilayer studded with viral glycoproteins; this process does not necessarily kill the cell immediately, allowing chronic infection. Influenza viruses bud from the apical surface of respiratory epithelial cells, releasing progeny virions into the airway lumen from where they can be inhaled by new hosts. HIV buds primarily from T cells, and the viral enzyme protease matures the budded particles into infectious virions.

Why Viruses Are Hard to Kill

The challenge of antiviral drug development stems from a fundamental problem: viruses replicate inside host cells using host machinery. Drugs that disrupt viral replication must be selective — they must inhibit viral processes without fatally interfering with host cell functions. This selectivity is far more challenging to achieve than for antibiotics, which target bacterial structures (cell walls, ribosomes, DNA topoisomerases) that have no counterpart in human cells.

The viral processes most amenable to selective inhibition are those encoded by viral genes with no close human counterpart: viral polymerases (HIV reverse transcriptase, viral DNA polymerases, RdRp), viral proteases, and viral surface proteins. HIV treatment exemplifies this approach: highly active antiretroviral therapy (HAART) combines inhibitors of reverse transcriptase, integrase, and protease — three distinctly viral enzymes — to suppress replication below detectable levels. But rapid mutation is another barrier: RNA viruses in particular have error-prone polymerases (they lack proofreading) and generate enormous genetic diversity. HIV generates approximately one mutation per genome per replication cycle, producing a vast swarm of variants from which drug-resistant mutants can be rapidly selected. Using multiple drugs simultaneously — combination therapy — prevents resistance by requiring multiple simultaneous mutations to escape all drugs at once, an extremely unlikely event. The evolution of vaccine and antiviral strategies mirrors the evolutionary arms race between viruses and immune systems that has been ongoing for as long as life has existed.