‘Tis the season… flu season, that is. Symptoms like a runny nose, scratchy throat, or fever are all telltale signs, but there is another way to know your body might soon be down for the count. If you drag yourself to the doctor, she will likely feel behind your ears and under your jaw and declare that your glands are swollen.

Those glands, also called lymph nodes, are a group of small cells that both catch and break down the viruses that get into the body. There are two types of lymph node cells: lymphocytes and macrophages. The first captures viral illnesses and the second destroys them.

Lymph nodes are just one part of your body’s lymphatic system, which includes lymphatic vessels, a network of tubes that carries white blood cells that help fight infection, and lymph fluid, which travels through the vessels and gets squeezed out of capillaries in the cardiovascular system and drained away. The lymphatic system runs throughout your body and is present in your bone marrow, spleen, and as scientists recently discovered, even your brain.

The body has hundreds of lymph nodes, clustered together to take care of drainage for different areas of the body. Along with the lymph nodes under the jaw and on either side of the neck, there are also lymph nodes in the armpits, on either side of the groin, and above the collarbone.

For an infection like strep throat, the lymph nodes in your head and neck might swell since they are located closest to the infection. This is a sign that your body is working hard to fight off the infection, and when it’s successfully fought off, your lymph nodes will return to their normal size.

Your body fights infections constantly, and while many cause no long-term harm, some infections that cause your lymph nodes to swell can be more serious. Here are four main types of illnesses that cause your lymph nodes to swell:

Everyday Illness
These illnesses generally require no more than rest and/or a course of antibiotics to knock them out — think a sore throat or strep. Swollen lymph nodes in your head and neck could also indicate an ear infection or an abscessed tooth. Less common, but still treatable, are measles or mononucleosis, the latter of which is the “kissing disease” that often makes the rounds on college campuses.

Out-of-the-ordinary illness
These illnesses are less likely to occur, but diseases like tuberculosis and syphilis have been on the rise. Since tuberculosis is caused by a bacterial disease, the lymph nodes would be working to try to fight it off. Syphilis would also fall into this category. But, don’t get too worried, both are curable diseases.

Chronic illness

Swollen glands could also be a result of a chronic, long-term illness. Women with the chronic inflammatory disease lupus, the bacterial infection Lyme disease, or rheumatoid arthritis might notice swelling of lymph nodes all over their body rather in just one area. Human Immunodeficiency Virus (HIV) also falls into this category.

Life-threatening Illness
You may not know it, but cancer is, in many ways, the disease most associated with the lymph nodes. Cancer that begins in the lymph nodes is called lymphoma, as in non-Hodgkin’s lymphoma. Remember those two types of cells in lymph nodes? One, lymphocytes, causes non-Hodgkin’s lymphoma when the body produces too many abnormal lymphocytes. Even if cancer doesn’t begin in the lymph nodes, it can often spread there from somewhere else. For instance, leukemia is a cancer of your body’s blood forming system, which includes bone marrow and the rest of the lymphatic system. For women, in particular, swollen lymph nodes in the underarms can indicate serious illness. Those lymph nodes are often the first place breast cancer spreads.

If your lymph nodes are swollen, there might be pain when you touch the area. You can relieve some of the immediate discomfort by using a warm compress and taking a pain reliever.

Remember, though, swollen lymph nodes are an indication of infection in your body, so you want to get to the bottom of what the infection could be. A sore throat and runny nose might just require vitamin C, tea, and some time on the couch. If, however, your lymph nodes are swollen in more than one area, you have a fever, or your lymph nodes have been swollen for more than two weeks and continue to enlarge, consult your doctor to find out if something more serious might be going on.

Camille Acker is a freelance writer living in Chicago and is a co-founder of The Spinsters Union. She is the proud owner of many, many books. See more at camilleacker.com.