Shingles is primarily caused by the varicella-zoster virus, which enters the human bloodstream through the respiratory tract mucosa, leading to chickenpox or a latent infection. The virus then lies dormant in the nerve ganglia of the body. Certain triggers, such as trauma, fatigue, weakness after illness, malignant tumors, or the use of immunosuppressants, can decrease the body's resistance and activate the dormant virus, resulting in shingles.