Viruses can hijack living cells to reproduce, cause diseases ranging from the common cold to cancer, hide dormant in your body for decades, reshape entire ecosystems, and even serve as tools in modern medicine. They infect every form of life on Earth, and roughly 8% of the human genome is made up of ancient viral DNA. What viruses “do” extends far beyond making you sick.
How Viruses Take Over Cells
Viruses cannot reproduce on their own. They lack the cellular machinery to copy their genetic material or build proteins, so they must commandeer a living cell to do it for them. The process follows a consistent pattern across nearly all viruses, though the details vary by species.
First, a virus attaches to the surface of a host cell by binding to a specific receptor, much like a key fitting into a lock. This is why particular viruses only infect particular cell types. After attachment, the virus enters the cell in an energy-dependent process, meaning the cell must be metabolically active. Once inside, the virus sheds its outer protein shell (a step called uncoating), exposing its genetic material. The cell’s own machinery then reads that genetic material and begins producing viral proteins and copying the viral genome. New virus particles assemble inside the cell, and finally they escape. Some viruses slip out by budding through the cell membrane, stealing a piece of it as a lipid envelope in the process. Others simply rupture the cell, killing it and spilling hundreds or thousands of new virus particles into the surrounding tissue.
Infecting Every Domain of Life
Viruses don’t just infect humans. They target organisms across all three domains of life: bacteria, archaea, and eukaryotes (which includes animals, plants, fungi, and protists). Bacteriophages, the viruses that infect bacteria, are the most abundant biological entities on the planet. They turn up in deserts, hot springs, oceans, soil, sewage, and the human gut. Some can withstand extreme dryness, temperature swings, and highly acidic or alkaline conditions while remaining infectious for years.
The oceans are an especially dense viral habitat, teeming with phages that infect marine bacteria and archaea alongside viruses that target algae, fish, and invertebrates. These marine viruses kill enormous numbers of microorganisms daily, driving microbial turnover and recycling nutrients like carbon, nitrogen, and phosphorus back into the water column. This viral activity fundamentally shapes ocean ecosystems and plays a significant role in global carbon cycling.
Types of Infections in Humans
Not all viral infections behave the same way. They fall into three broad categories, and understanding the differences explains why some illnesses vanish in days while others last a lifetime.
Acute infections are the most familiar. A respiratory virus, a stomach bug, or a case of the flu hits hard, triggers an immune response, and resolves within days to two weeks. Your immune system clears the virus, and it’s gone.
Chronic infections persist for months or years because the virus continues replicating at low levels despite the immune response. Hepatitis B and C are classic examples, capable of quietly damaging the liver over decades. HIV is a lifelong chronic infection that, without treatment, progressively weakens the immune system.
Latent infections are perhaps the most surprising. Certain viruses settle into specific cell types and go silent, producing no symptoms and not actively replicating. Herpes simplex and varicella-zoster (the virus behind chickenpox) hide in sensory nerve cells. Epstein-Barr virus tucks itself into memory immune cells. HIV can establish silent reservoirs in resting immune cells, where the virus integrates into the cell’s DNA but stops producing new copies because the host cell has returned to a quiet state. These dormant viruses can reactivate years or even decades later. Chickenpox reappearing as shingles in older adults is a textbook example.
Causing Cancer
Seven viruses are recognized as direct contributors to human cancer. They include HPV (cervical and throat cancers), hepatitis B and C (liver cancer), Epstein-Barr virus (certain lymphomas and nasopharyngeal cancer), the virus behind Kaposi’s sarcoma, a virus linked to a rare type of leukemia, and Merkel cell polyomavirus (a skin cancer). Together, viral infections cause over 1.4 million cancer cases per year, accounting for roughly 15 to 20% of the global cancer burden. That makes viruses one of the most significant preventable causes of cancer worldwide, since vaccines exist for HPV and hepatitis B.
Damaging Agriculture
Viruses are a major threat to the global food supply. Plant diseases caused by all pathogens combined cost the global economy an estimated $220 billion annually, and viruses account for nearly half of that, more than $30 billion per year. Rice alone, cultivated in 100 countries and feeding nearly half the world’s population, faces annual losses of $1.5 billion from multiple insect-transmitted viruses. A single virus affecting potatoes destroys roughly 20 million tons of crop each year. Unlike bacterial or fungal infections, viral plant diseases have no direct chemical treatment, making prevention through resistant crop varieties and insect vector control the primary strategy.
Leaving DNA in the Human Genome
About 8% of your DNA consists of sequences left behind by ancient retroviruses that infected our ancestors’ reproductive cells millions of years ago. These stretches of viral code, called endogenous retroviruses, were passed down through generations and are now a permanent part of every human genome.
Most of these viral remnants are inactive, but some have been repurposed for essential biological functions. Two proteins originally encoded by endogenous retroviruses, syncytin-1 and syncytin-2, are critical for forming the placenta. They drive the cell fusion that creates the layer of tissue at the boundary between mother and fetus. Without these co-opted viral proteins, human pregnancy as we know it wouldn’t work.
Protecting the Body
Not all viruses are harmful. The human body harbors a diverse community of viruses, collectively called the virome, and some of them appear to play protective roles. In the gut, bacteriophages concentrate in the mucus lining of the intestinal wall, where they act as an additional barrier against harmful bacteria trying to breach the tissue beneath.
Animal studies have shown that certain viruses can compensate for the absence of beneficial gut bacteria. Mice raised in sterile environments or treated with antibiotics develop abnormalities in their intestinal lining and immune defenses. Infection with a common, typically harmless mouse virus reverses these problems and even protects against intestinal injury from chemical irritants or bacterial infection. Other research has found that a herpesvirus in mice boosts long-term immune activation, protecting the animals against subsequent infection by dangerous bacteria, including the one that causes plague.
Serving as Medical Tools
Scientists have turned the viral talent for entering cells and delivering genetic material into a medical advantage. Modified viruses, stripped of their ability to cause disease, now serve as delivery vehicles for gene therapy. They carry corrective genes into a patient’s cells to treat conditions caused by faulty or missing genes.
The FDA has approved more than a dozen gene therapies that use viral vectors to deliver genes directly into the body. These treat conditions including an inherited form of blindness, spinal muscular atrophy in infants, and certain types of hemophilia. One modified herpes virus is even approved to treat melanoma by being injected directly into tumors, where it replicates inside cancer cells and destroys them.
Viruses also underpin a growing class of cancer immunotherapies. In CAR-T cell therapy, doctors remove a patient’s immune cells, use a modified virus to insert a gene that programs those cells to recognize cancer, and then infuse them back into the patient. Multiple CAR-T therapies are now approved for blood cancers like lymphoma and leukemia, with over a dozen products on the market. The same viral delivery approach is being used for inherited blood disorders like sickle cell disease and certain metabolic conditions in children.