Almost any infection can cause swollen lymph nodes. The most common culprits are everyday viral infections like colds, flu, and mono, followed by bacterial infections such as strep throat and staph skin infections. Lymph nodes swell because they are active filtering stations where your immune system fights off invaders, and that increased activity causes them to enlarge, sometimes noticeably.
Why Infections Make Lymph Nodes Swell
Lymph nodes are small, bean-shaped structures scattered throughout your body, connected by a network of vessels that carry fluid from your tissues. When bacteria or viruses enter your body, those pathogens (or immune cells that have captured them) travel through lymphatic vessels to the nearest lymph node. Once there, the node ramps up production of immune cells and antibodies to fight the infection. This surge of cellular activity, along with increased fluid flow, is what makes the node physically enlarge. The swelling is a sign your immune system is doing its job.
The location of the swelling often points to where the infection is. Nodes in your neck swell during throat and respiratory infections. Nodes in your armpit respond to infections in your arm or chest. Nodes in your groin react to infections in your legs or genital area. When an infection is widespread, like mono or HIV, nodes may swell in multiple locations at once.
Viral Infections
Viral infections are the single most common reason lymph nodes swell, especially in children. A routine upper respiratory infection (the common cold) frequently causes small, tender nodes along the sides of the neck. These are typically bilateral, mobile under the skin, and clearly defined to the touch.
Beyond colds, several viruses are well known for producing noticeable node swelling:
- Epstein-Barr virus (mono): Classic presentation is swollen nodes in the back of the neck along with sore throat and deep fatigue. More common in school-age children and young adults.
- Cytomegalovirus (CMV): Causes a mono-like illness with swollen nodes, often in adolescents and adults.
- Herpes simplex virus: Can cause swollen nodes near the site of a cold sore (neck nodes) or genital outbreak (groin nodes). First-time infections tend to produce more swelling than recurrences.
- HIV: Generalized swelling of lymph nodes in multiple body areas is one of the early signs of HIV infection.
Coxsackievirus, which causes hand-foot-and-mouth disease, is another common trigger, particularly in young children.
Bacterial Infections
Bacterial infections tend to cause more intense, localized swelling compared to viruses. Two bacteria account for the majority of cases: Staphylococcus aureus (staph) and Streptococcus pyogenes (strep). A child with a skin wound that becomes infected, for example, will often develop swollen, tender nodes in the nearest drainage area. Strep throat commonly causes enlarged, painful nodes on the front of the neck.
Strep bacteria have a particularly strong association with lymphatic problems. Up to a quarter of invasive strep infections present as bloodstream infections without an obvious source, partly because the bacteria can travel through the lymphatic system itself. Staph infections in the lymph nodes can actually interfere with normal immune responses, which sometimes prolongs the illness.
Other bacterial causes include dental abscesses (swollen nodes under the jaw), mycobacterial infections including tuberculosis (chronic node swelling, often in the neck), and cellulitis, a skin infection that typically causes groin or armpit node swelling depending on where it occurs.
Cat Scratch Disease and Other Zoonotic Infections
Cat scratch disease, caused by Bartonella henselae, is one of the more distinctive infectious causes of lymph node swelling. A cat scratch or bite introduces the bacteria, and within one to three weeks a node near the wound swells, sometimes dramatically. If a cat scratches your hand, you might notice a swollen node in your armpit. The bacteria spread among cats through fleas, and the infection occurs worldwide.
Tularemia, caused by Francisella tularensis, produces swollen nodes after contact with infected rabbits, rodents, or tick bites. It’s found throughout the Northern Hemisphere. Brucellosis, contracted through unpasteurized dairy products or direct animal contact, can also cause significant node swelling. Toxoplasmosis, a parasitic infection usually acquired from undercooked meat or cat litter, is another recognized cause, often producing painless cervical node swelling.
Sexually Transmitted Infections
Several STIs cause swollen lymph nodes in the groin. Lymphogranuloma venereum, caused by certain strains of Chlamydia, produces painful, one-sided groin swelling that can become fluctuant or even rupture. These swollen masses, called buboes, are a hallmark of the disease.
Genital herpes, particularly during a first outbreak, often triggers groin node swelling along with fever, headache, and malaise. Recurrent outbreaks are less likely to cause noticeable node changes. Syphilis produces painless groin nodes during its primary stage. Chancroid, a less common bacterial STI, causes painful genital ulcers paired with tender, sometimes draining groin nodes.
What Swollen Nodes Feel Like by Cause
The way a swollen node feels can offer clues about what’s behind it. Nodes swollen from viral infections are typically soft, mobile, and not especially painful. You can roll them under your fingers. They’re often on both sides of the neck and have clear edges.
Bacterial infections tend to produce nodes that are warm, tender, and sometimes red on the overlying skin. They’re usually on one side and may feel slightly firmer. In severe cases, a node can fill with pus and become an abscess.
Nodes that are hard, painless, and feel fixed to the tissue underneath are more concerning for non-infectious causes like lymphoma or metastatic cancer. Nodes above the collarbone also carry higher significance for malignancy. Age matters too: in children, swollen nodes are almost always reactive to infection, while in adults over 40, persistent swelling deserves closer evaluation.
Swollen Nodes in Children
Reactive swelling from viral infections is the most common cause of enlarged neck nodes in children, and it is so frequent that palpable lymph nodes in healthy children are considered normal. Bilateral neck swelling during a cold or ear infection is the most typical scenario.
Unilateral (one-sided) neck swelling in children is more often bacterial, usually staph or strep. Atypical mycobacterial infection, also called non-tuberculous mycobacterial infection, is the most common cause of chronic neck node swelling in children ages one to five. It typically appears as a painless, slowly growing lump that doesn’t respond to standard antibiotics. EBV and CMV infections tend to affect older, school-age children and adolescents.
How Long the Swelling Lasts
Most infection-related lymph node swelling resolves within two to three weeks as the infection clears. In children especially, nodes may shrink but remain firm and visible for several weeks afterward. This lingering firmness is normal and doesn’t necessarily mean the infection is still active.
Nodes that remain enlarged for more than a month, continue to grow, or are accompanied by unexplained weight loss, night sweats, or persistent fever warrant medical evaluation. The combination of how the node feels, where it is, how long it’s been present, and your other symptoms helps determine whether further testing is needed.
Location and What It Suggests
The body region where you notice swelling narrows the list of likely infections considerably:
- Neck (cervical): Upper respiratory viruses, strep throat, mono, dental infections, ear infections, mycobacterial infections
- Behind the ear: Scalp infections, rubella, ear infections
- Armpit (axillary): Staph skin infections on the hand or arm, cat scratch disease, brucellosis
- Groin (inguinal): STIs, cellulitis of the leg or foot, localized skin infections
- Multiple locations (generalized): Mono, CMV, HIV, toxoplasmosis, secondary syphilis
Swelling that stays in one area and matches a nearby infection site is usually straightforward. Generalized swelling affecting nodes in two or more separate body regions points toward a systemic infection and typically calls for blood work to identify the cause.