A virus infects a cell through a multi-step process: it latches onto specific proteins on the cell’s surface, crosses the cell membrane, sheds its protective shell, and hijacks the cell’s machinery to copy itself. Each step involves precise molecular interactions, and the details vary depending on the type of virus. Here’s how the full process works, from first contact to the release of new virus particles.
Attachment: Finding the Right Cell
Infection begins when a virus bumps into a cell that displays the right surface proteins. Viruses carry their own surface proteins (often called “spikes”) that fit into specific receptors on host cells the way a key fits a lock. This match determines which cells a virus can infect. HIV, for instance, uses a spike protein called gp120 to grab onto a protein called CD4, which sits mainly on immune cells called T cells. That’s why HIV targets immune cells and not, say, lung cells. SARS-CoV-2 and related coronaviruses use their spike protein to bind a different set of receptors, which is why they target the respiratory tract instead.
This receptor requirement is the main reason viruses are picky about which cells, tissues, and even species they can infect. Rhinoviruses (the common cold) need a protein called ICAM-1 to get in. Hepatitis B thrives in the liver partly because liver cells are rich in specific factors the virus needs for replication. If a cell doesn’t display the right receptor, the virus simply can’t attach, and infection never starts.
Some viruses need more than one receptor. HIV first binds CD4, then must also grab a second protein, either CCR5 or CXCR4, before it can proceed. This two-step handshake adds another layer of specificity and is one reason HIV infects only certain subtypes of immune cells.
Entry: Getting Inside the Cell
Once attached, a virus needs to cross the cell membrane. There are two main strategies, and which one a virus uses depends largely on whether it has a lipid envelope (a fatty outer layer stolen from a previous host cell) or not.
Membrane Fusion
Enveloped viruses like HIV can fuse directly with the cell’s outer membrane. After binding its receptors, the virus’s spike proteins undergo a shape change that pulls the viral envelope and the cell membrane together until they merge into one continuous sheet. The virus’s genetic cargo then spills into the cell’s interior. This whole process happens at the cell surface and takes seconds to minutes.
Endocytosis
Many viruses, including influenza, trick the cell into swallowing them whole. After attachment, the cell membrane wraps around the virus particle and pinches off to form a small bubble (called a vesicle) inside the cell. The cell essentially pulls the virus inward as if it were absorbing a nutrient. Influenza has roughly 400 spike proteins on its surface that bind to sugar molecules on the cell, promoting this wrapping process. Once the virus is sealed inside the vesicle, the cell gradually acidifies its interior. That drop in pH triggers the viral spikes to change shape and fuse with the vesicle wall, releasing the viral contents into the cell’s main compartment.
Non-Enveloped Viruses
Viruses without a lipid envelope, like HPV and many cold viruses, face a tougher challenge because they can’t fuse membranes. These viruses typically enter through endocytosis, then use specialized proteins to punch through or slip across the vesicle membrane. HPV, for example, carries a protein with a short sequence that acts like a cell-penetrating drill bit. It inserts into the vesicle membrane with help from a host protein called gamma-secretase, and if either of those components is missing, the virus gets trapped inside the vesicle and infection fails. Some non-enveloped viruses, like the polyomavirus SV40, take an even more unusual route: after endocytosis, they travel all the way to a cellular compartment called the endoplasmic reticulum and remodel its membrane to escape into the cell interior.
Uncoating: Releasing the Genetic Material
Once inside the cell, the virus still needs to shed its protein shell (the capsid) to free its genetic material. This step is called uncoating, and it’s triggered by signals from the cell’s own environment. The drop in pH inside vesicles is one common trigger, causing the capsid to change shape and fall apart. Cellular enzymes also play a role: the cell’s protein-recycling systems can chew up parts of the viral coat, and motor proteins that normally haul cargo along the cell’s internal scaffolding can physically pull the capsid apart. The result is naked viral DNA or RNA, now loose inside the cell and ready to be copied.
Replication: Copying the Viral Genome
This is where the real hijacking begins. The virus commandeers the cell’s own machinery to copy its genetic material and build new viral proteins. How it does this depends on whether the virus carries DNA or RNA.
DNA viruses generally move their genome into the cell’s nucleus, where they can use many of the same enzymes the cell already uses to copy DNA and read genes. Smaller DNA viruses lean heavily on the host’s own copying equipment. Larger ones, like herpesviruses, bring some of their own.
RNA viruses face a different problem: human cells have no built-in way to copy RNA from an RNA template. So these viruses must carry or quickly produce their own copying enzyme, called an RNA-dependent RNA polymerase. Positive-sense RNA viruses (like the coronavirus behind COVID-19) have a shortcut: their genome can be read directly by the cell’s protein-making machinery the moment it enters the cytoplasm, producing viral proteins including the copying enzyme almost immediately. Negative-sense RNA viruses (like Ebola and influenza) carry their copying enzyme pre-packaged inside the virus particle, because their genome needs to be converted into a readable form first. Both types replicate in the cytoplasm, outside the nucleus.
Retroviruses like HIV use yet another approach. They carry an enzyme called reverse transcriptase that converts their RNA genome into DNA. That DNA then travels to the nucleus and integrates into the cell’s own chromosomes, effectively becoming part of the cell’s genetic code. From there, the cell reads and copies the viral instructions as if they were its own, which is why HIV infections are lifelong.
Assembly and Release: Building New Viruses
After the cell has produced copies of the viral genome and manufactured viral proteins, new virus particles are assembled. The specific proteins and genetic copies self-assemble into new capsids, often in the cytoplasm. What happens next depends on the virus type.
Enveloped viruses like HIV use a process called budding. Newly assembled viral cores migrate to the cell membrane, where viral surface proteins have already been inserted. The core pushes outward against the membrane, wrapping itself in a piece of the cell’s own lipid layer as it pinches off. Each new virus buds out individually, and the cell can survive for a while, continuously releasing new particles. For HIV, this budding happens at the outer cell membrane. Other viruses bud into internal compartments first and are then shuttled out.
Non-enveloped viruses typically exit by a blunter method: the cell fills up with new virus particles until it bursts open (lysis), spilling hundreds or thousands of copies into the surrounding tissue. Cell lysis happens partly because the virus disrupts normal cell functions, and partly because many viruses actively trigger the cell’s self-destruct program (apoptosis). Either way, the host cell is destroyed.
How Your Cells Fight Back
Cells aren’t defenseless. They have internal alarm systems designed to detect viral invaders and slow them down. Three major sensor pathways patrol for signs of infection. Toll-like receptors sit inside cellular compartments and detect unusual RNA or DNA that shouldn’t be there. A second set of sensors in the cytoplasm specifically recognizes double-stranded RNA, a hallmark of viral replication that normal cells rarely produce. A third sensor detects stray DNA floating in the cytoplasm, which is another red flag for viral activity.
When any of these sensors is triggered, the cell activates a cascade that leads to the production of interferons, signaling molecules that do two things at once. They switch on antiviral defenses inside the infected cell itself, slowing down viral replication. And they act as a chemical alarm to neighboring cells, warning them to ramp up their own defenses before the virus reaches them. This interferon response is one of the earliest and most important barriers to viral spread, and it kicks in hours before the broader immune system (antibodies, killer T cells) even knows an infection is underway.
Viruses, in turn, have evolved countless tricks to evade or suppress these defenses, which is part of why some viruses cause mild illness while others can be devastating. The outcome of any infection is essentially a race between the virus’s ability to replicate and the cell’s ability to sound the alarm.