The most common infections that cause swollen lymph nodes are everyday viral illnesses, especially upper respiratory infections like colds and flu. But a wide range of bacterial, parasitic, and sexually transmitted infections can also trigger node swelling, sometimes in patterns that point directly to the cause. The location of the swelling, how many nodes are involved, and how long they stay enlarged all offer clues about what’s going on.
Why Infections Make Lymph Nodes Swell
Lymph nodes are small, bean-shaped filters stationed throughout your body, with clusters in your neck, armpits, chest, abdomen, and groin. When you pick up an infection, bacteria or viral particles travel through your lymphatic vessels and enter nearby nodes. Immune cells inside the node detect these invaders, activate, and begin multiplying rapidly to mount a defense. That burst of immune cell production is what physically enlarges the node, sometimes making it tender to the touch. The swelling is a sign your immune system is working, not a sign something has gone wrong with the node itself.
Viral Infections
Viruses are the single most common reason for swollen lymph nodes, particularly in the neck. A standard cold or upper respiratory infection, caused by rhinovirus, parainfluenza, influenza, respiratory syncytial virus, coronavirus, or adenovirus, will often produce mildly swollen, tender nodes along the sides of your neck. These typically shrink back to normal within a couple of weeks as the illness resolves.
Several other viruses are known to cause more noticeable or longer-lasting node swelling:
- Epstein-Barr virus (EBV), the cause of mono, is one of the classic culprits. It typically produces significant swelling in the back of the neck along with severe fatigue and sore throat.
- Cytomegalovirus (CMV) causes a mono-like illness with swollen nodes and fatigue, though often without the intense sore throat.
- HIV can cause widespread swollen nodes during the initial acute infection, often alongside a flu-like illness and rash. Persistent generalized swelling lasting months is also a hallmark of later-stage HIV.
- Herpes simplex virus (HSV) tends to cause localized swelling near the site of an outbreak, such as groin nodes during a genital herpes flare.
- Varicella-zoster virus, which causes chickenpox and shingles, can produce generalized node swelling during chickenpox or localized swelling near a shingles rash.
- Rubella and measles both cause swollen nodes, with rubella particularly known for enlarging nodes behind the ears.
Bacterial Infections
Bacterial infections tend to cause more dramatic, localized swelling compared to viruses. The node often becomes visibly enlarged (over 2 cm), red, warm, and quite painful.
Staph and strep bacteria are responsible for 40% to 80% of cases of acute lymph node infection in children. Strep throat is a textbook example: it produces tender, swollen nodes at the front of the neck, draining from the infected throat. Staph infections, including skin abscesses and wound infections, cause swelling in whatever node cluster is closest to the infected area. MRSA, a drug-resistant form of staph, is increasingly common in these cases. When bacteria overwhelm a lymph node, it can fill with pus, a condition called suppurative lymphadenitis. This causes intense pain, redness over the skin, fever, and a general feeling of being unwell.
Tuberculosis remains a major cause of swollen lymph nodes globally, especially in sub-Saharan Africa, Southeast Asia, and the Indian subcontinent. TB-related node swelling tends to develop slowly, often in the neck, and the nodes may feel firm or matted together.
Syphilis, a sexually transmitted bacterial infection, causes swollen nodes in two distinct phases. During the primary stage, painless groin nodes swell near the site of the initial sore. During the secondary stage, nodes throughout the body can enlarge as the infection spreads systemically.
Cat-Scratch Disease
Cat-scratch disease deserves its own mention because it’s common and often puzzling if you don’t connect the dots. It’s caused by a bacterium transmitted through a cat scratch or bite, most often from kittens. A small bump forms at the scratch site within three to five days. Then, one to two weeks later, lymph nodes near the scratch swell noticeably. If the scratch was on your hand, your armpit or elbow-area nodes swell. If it was on your face, neck nodes enlarge. About 25% of cases involve neck nodes. Early symptoms include low-grade fever, fatigue, muscle aches, and poor appetite. The swelling can persist for weeks or even months before resolving on its own.
Parasitic Infections
Toxoplasmosis, caused by a parasite found in undercooked meat and cat feces, is the most common parasitic cause of swollen lymph nodes in developed countries. It typically causes painless swelling in neck nodes and may come with mild fatigue and muscle aches. Most healthy people recover without treatment, but the infection can be serious for pregnant women and people with weakened immune systems. Chagas disease, transmitted by a type of insect found in Central and South America, can also cause node swelling during its early phase.
Rare Zoonotic Infections
Some uncommon infections transmitted from animals or insect bites cause distinctive patterns of swollen nodes. Tularemia, sometimes called “rabbit fever,” spreads through tick bites, contact with infected rabbits or rodents, or contaminated water. It causes dramatic swelling of nodes near the site of exposure. Bubonic plague, caused by bacteria carried by fleas on rodents, produces massively swollen, painful groin or armpit nodes called buboes. Brucellosis, which spreads through unpasteurized dairy products or direct contact with livestock, can cause widespread node swelling along with recurring fevers. These infections are rare in most of the world but worth knowing about if you have relevant animal or outdoor exposures.
What the Location Tells You
Your body drains lymph fluid through predictable routes, so the location of swollen nodes often points to where the infection started. Swollen neck nodes suggest an infection in the head, mouth, throat, or upper respiratory tract. Armpit nodes swell in response to infections in the arm, hand, or chest wall. Groin nodes react to infections in the leg, foot, or genital area. When nodes swell in just one area, the cause is usually a localized infection nearby.
Generalized swelling, where nodes enlarge in two or more separate regions of the body, points to a systemic infection affecting the whole body. HIV, mono, CMV, syphilis, and toxoplasmosis are the most common infectious causes of this pattern. Generalized swelling generally warrants more investigation than a single swollen node in your neck during a cold.
How Long the Swelling Lasts
Most infection-related lymph node swelling resolves within two to four weeks as the infection clears. With proper treatment for bacterial infections, the swelling often improves even faster, though it can take additional time for the node to return fully to its normal size. In some cases, a previously infected node stays slightly enlarged for weeks or months after the infection is gone. This is normal and doesn’t necessarily mean the infection is still active.
Nodes that continue growing over several weeks, feel hard or rubbery, are painless, are fixed in place rather than movable, or are larger than about 2 centimeters deserve medical evaluation. Painful, tender nodes are actually more reassuring in most cases, since infection-related swelling tends to hurt while other causes often don’t. Nodes that haven’t started shrinking after four to six weeks, or that are accompanied by unexplained weight loss, night sweats, or persistent fevers, should be assessed promptly.
How Doctors Identify the Cause
In most cases, your doctor can identify the likely infection from your symptoms, the node’s location, and a physical exam. A sore throat with tender neck nodes in a teenager points toward mono or strep and can be confirmed with a rapid strep test or a mono test. A swollen armpit node with a history of a recent cat scratch suggests cat-scratch disease, which can be confirmed with a blood test for antibodies.
When the cause isn’t obvious, blood tests can check for specific infections: HIV, syphilis, toxoplasmosis, EBV, and CMV all have reliable blood-based tests. If a node appears to contain pus, a sample can be drawn with a needle and tested to identify the exact bacteria involved and which antibiotics will work against it. In cases where infection isn’t the clear explanation, a node biopsy, where part or all of the node is removed and examined under a microscope, can help rule out other causes.