Infection happens when a germ enters your body, multiplies, and triggers a reaction from your immune system. That reaction, not the germ itself, is what you experience as being sick. The process follows a predictable sequence, from how the pathogen finds you to how your body fights back.
What Counts as an Infection
There’s an important distinction between germs being present and an actual infection. Colonization is when germs live on or in your body without causing symptoms. You can carry bacteria on your skin or in your nose for years and never get sick from them, though you can still pass those germs to others. An infection only occurs when the germs increase in number and your body mounts a response: inflammation, fever, pain, or tissue damage.
The Chain of Infection
Every infection follows a six-link chain. Break any single link and the infection can’t happen.
- Pathogen: The disease-causing organism, whether a virus, bacterium, fungus, or parasite.
- Reservoir: The environment where the pathogen lives and multiplies. This can be a person, an animal, soil, or water.
- Portal of exit: How the pathogen leaves its reservoir. Respiratory infections leave through coughs and sneezes. Gastrointestinal pathogens leave in feces. Others exit through blood, breast milk, or semen.
- Mode of transmission: How the pathogen travels to a new host, whether through direct contact, airborne particles, contaminated surfaces, food and water, or insect bites.
- Portal of entry: The opening where the pathogen gets in. Common entry points include the respiratory tract (mouth, nose, lungs), mucous membranes, breaks in the skin, and blood. Influenza exits one person’s respiratory tract and enters the next person’s respiratory tract. Many gut infections follow a fecal-oral route: pathogens leave in feces, get carried on unwashed hands to food or water, and enter a new host through the mouth.
- Susceptible host: A person whose defenses can’t stop the pathogen from establishing itself.
How Viruses Infect Cells
Viruses can’t reproduce on their own. They hijack your cells to make copies of themselves, following seven stages in a fixed order: attachment, penetration, uncoating, replication, assembly, maturation, and release.
It starts with attachment. A virus carries specific proteins on its surface that lock onto matching receptors on your cells, the way a key fits a lock. These receptors are normal molecules your cells use for everyday functions. The cold virus (rhinovirus), for example, latches onto a protein your cells use to stick to each other. Influenza binds to sugar molecules found on the tips of your cell’s carbohydrate chains. This specificity is why certain viruses only infect certain tissues: a liver virus can’t infect your lungs if the right receptors aren’t there.
Once attached, the virus quickly pushes inside through a process called penetration. This step requires energy, but the virus doesn’t supply it. Your own cell provides the power, often through a normal process called endocytosis, where the cell membrane wraps around the virus and pulls it in like swallowing a package. Speed matters here because the virus needs to get inside before it gets swept away by mucus or other defenses.
Inside the cell, the virus sheds its outer shell (uncoating) and releases its genetic material. It then commandeers your cell’s protein-making machinery to produce copies of its own genes and build new viral proteins. These components are assembled into new virus particles, which mature into infectious copies and burst out of the cell, often destroying it in the process. Each released virus particle can then infect another cell, and the cycle repeats exponentially.
How Bacteria Cause Disease
Bacteria work differently from viruses. They’re complete living organisms that can grow on their own, and they use a toolkit of “virulence factors” to invade your body, cause damage, and dodge your defenses.
The first step is adhesion. Many disease-causing bacteria have tiny hair-like projections called pili that anchor them to the cells lining your throat, gut, or urinary tract, preventing them from being flushed away. Once anchored, some bacteria invade directly into your cells. Salmonella, for instance, can push its way inside the cells lining your intestines, though it doesn’t need to be inside cells to survive.
Bacteria cause damage in two main ways. Some release toxins, proteins that poison your cells or disrupt normal body functions. Others carry molecules on their outer surface that trigger massive inflammation. The outer coating of certain bacteria (called gram-negative bacteria) contains a substance that, when released, can cause fever, dangerous drops in blood pressure, widespread inflammation, and in severe cases, shock.
How Many Germs It Takes
Not every exposure leads to infection. The number of pathogen particles needed to infect 50% of exposed people is called the infectious dose, and it varies wildly between organisms and even between routes of exposure.
Some pathogens are astonishingly efficient. A common cold coronavirus (strain 229E) needs only about 13 viral particles delivered to the nose to infect half the people exposed. Rhinovirus, another cold virus, needs even fewer when delivered as nose drops: roughly 0.03 of a standard viral unit, meaning a tiny fraction of a single measurable dose is enough. Adenovirus type 4 needs just about half a viral unit when inhaled as an aerosol, but roughly 35 units when dripped into the nose. That difference highlights how much the route of entry matters. Breathing in fine aerosol particles tends to be a far more efficient way to get infected than having droplets land in your nose.
For influenza, the picture varies by strain. The H2N2 strain needed fewer than 3 viral units by aerosol to infect half the people exposed, while the H1N1 strain required about 1,000 units delivered as nose drops, and the H3N2 strain needed 10 million units by the same route. SARS-CoV-2, the virus behind COVID-19, was estimated to require about 100 particles to establish an infection.
What Determines If You Get Sick
Exposure to a pathogen doesn’t guarantee infection. Whether you actually get sick depends on several factors working together.
Genetics play a foundational role. Your cells may or may not carry the specific receptors a particular pathogen needs to latch onto. In animal studies, susceptibility to certain viruses comes down to a single gene that determines whether key cells can support viral growth. Your immune response genes, clustered in a region of your DNA called the major histocompatibility complex, control how strongly your immune system reacts to specific invaders, and this varies significantly from person to person.
Age matters in complex ways. Newborns are highly vulnerable because their immune systems are still maturing. They rely heavily on antibodies passed from their mother during pregnancy or through breast milk. If those maternal antibodies are missing, infants are especially susceptible during the first weeks of life. On the other hand, some infections are paradoxically worse in adults. Chickenpox is usually mild in children but can cause severe pneumonia in adults. Mumps in adults is more likely to cause complications.
Nutritional status, stress levels, and pre-existing immunity round out the picture. Someone who has already fought off a particular pathogen, or been vaccinated against it, carries antibodies and immune memory cells that can neutralize the invader before it gains a foothold.
Your Body’s First Line of Defense
The moment a pathogen breaches your outer barriers (skin, mucus, stomach acid), your innate immune system activates. This is the fast, nonspecific response that buys time while your body mounts a more targeted attack.
Specialized immune cells called macrophages and neutrophils are the first responders. They recognize molecular patterns common to many pathogens, latch onto the invader, and engulf it in a process called phagocytosis. The cell’s membrane wraps around the pathogen, pulls it inside, and then unleashes a battery of chemical weapons to destroy it.
Simultaneously, your complement system kicks in. This is a group of about 20 proteins circulating in your blood that work like a cascade of dominoes. When one protein detects a pathogen, it splits into fragments. The larger fragment binds to the pathogen’s surface, marking it for destruction and making it easier for immune cells to grab. The smaller fragments act as chemical signals, drifting outward to recruit more immune cells to the site.
These processes trigger inflammation: the redness, heat, swelling, and pain you feel at an infection site. Blood vessels in the area widen and become more permeable, flooding the tissue with immune cells and defensive proteins. Your cells release signaling molecules called cytokines and prostaglandins that amplify the response and, when the infection is serious enough, trigger a fever. While uncomfortable, inflammation is the visible sign that your body has detected the threat and is actively fighting it.