Lymphangitis is an inflammation or an infection of the lymphatic channels that occurs as a result of infection at a site distal to the channel. The most common cause of lymphangitis in humans is Streptococcus pyogenes (Group A strep), hemolytic streptococci, and in some cases, mononucleosis, cytomegalovirus, tuberculosis, syphilis, and the fungus Sporothrix schenckii. Lymphangitis is sometimes mistakenly called "blood poisoning". In reality, "blood poisoning" is synonymous with sepsis.

Lymphatic vessels are smaller than capillaries and tiny venules and are ubiquitous in the body. These vessels are fitted with valves to direct flow in only one direction. Fluid diffusing through the thin-walled small capillaries should be collected and the lymphatic system does just that: a fluid rich in protein, minerals, nutrients, and other substances useful for tissue growth. As well as essential nutrients, the lymphatic system can also transport or carry cancer cells, defective or damaged cells, and pathogens such as bacteria and viruses, as well as foreign bodies and organisms. The lymph nodes are found in close proximity to unique white blood cells that engulf or metabolize pathogens (bacteria and viruses) and defective or cancerous cells, preventing infections and malignant cancer cells from spreading.

Infection spreads out of the wound site to enter the lymphatic system. The wound may be small or it may be an abscess constantly feeding bacteria into the lymphatic system. After infection, lymph nodes enlarge. Ear, skin, nose, and eye infections can spread into the lymphatic system. Red streaks in the skin along the direction of regional lymph nodes indicate lymphatic involvement. Infection may spread within hours and can cause sepsis and death.