Signs & Symptoms of an Infected Wound and Why

Signs
Inflammation
• Pus
• Swelling
• Redness
• Heat in area around wound
• Fever (pyrexia)

• Swelling or tenderness in the lymph nodes (located under armpits, in groin, neck areas and behind ears)
• Odour

Symptoms
• Pain

Why
Inflammation:
• Pus: large numbers of neutrophils, tissue cells and dead pathogens
• Swelling: increased permeability of the blood vessels and migration of leukocytes
• Redness: increased blood volume (due to vasodilation, where blood vessels become wider)
• Heat: increased blood volume and endogenous pyrogens
• Fever: endogenous pyrogens are cytokines (chemical signals) produced by macrophages, skin, endothelial (inside blood vessels), epithelial and glial (CNS) cells. Pyrogens signal to the hypothalamus to increase the temperature of the body. This allows infection to recover quicker because immunological reactions are sped up.
• Swelling in lymph nodes:
-Lymph nodes filter the lymphatic fluid and have special cells that can trap bacteria that are traveling through the body via the lymph fluid.
-Swelling of the lymph nodes may be due to the increased number of lymphocytes in the region as a response to the presence of an antigen or the large number of inflammatory cells filtering/passing through the lymph nodes.
• Odour: is due to organic acids secreted by anaerobic bacteria or necrotic tissue (which can be the home to pathogenic organisms)

• Pain: stimulation of neuronal pathways

Also, here's some background info about lymph nodes (just in case):

Lymph Nodes
Lymph nodes are a part of your lymphatic system, which is one of the body's barriers to infection and plays a role in the immune responses. When lymph nodes become swollen, it may signal an infection.
There are several groups of lymph nodes. The ones most frequently enlarged or swollen are in the neck, under the chin, in the armpits, and in the groin.
• The lymphatic system consists of nodes and ducts spread throughout the body. They bring the lymph (tissue fluids surrounding the cells) back into the circulation by way of the venous system. In the lymph, there is a concentration of infectious and other foreign substances (antigens).
• Lymph nodes are small clusters of cells, surrounded by a capsule. Ducts go in and out of them. The cells in lymph nodes are lymphocytes and macrophages (lymphocytes produce antibodies—protein particles that bind foreign substances including infectious particles—and macrophages digest the debris). They act as the "cleaner" cells of the body.
• The lymph nodes are a major site where foreign substances and infections interact with the cells of the immune system. A major cluster of the lymph nodes is the spleen, which, apart from other functions, also helps fight infections and responds to foreign substances.


http://www.emedicinehealth.com/swollen_lymph_glands/article_em.htm
http://www.worldwidewounds.com/1998/march/Odour-Absorbing-Dressings/odour-absorbing-dressings.html
http://www.bmj.com/cgi/reprint/332/7536/285.pdf