The inflammation is caused by infections spreading from nearby tissues. This can result in a clot forming within the vein, known as thrombophlebitis. This clot can disintegrate and spread infection to other parts of the body. If not caused by a pus-forming process, the vein can get blocked by the clot. Symptoms include pain and swelling in the affected area, with redness along the vein, which feels like a hard, sensitive strand.