Which virus is responsible for causing cold sores?

The virus responsible for causing cold sores is the herpes simplex virus type 1 (HSV-1). This virus typically manifests as small, painful blisters on or around the lips and is highly contagious. Once a person is infected with HSV-1, the virus remains dormant in the body and can reactivate under certain conditions, such as stress, illness, or exposure to sunlight, leading to recurrent outbreaks of cold sores.

The other viruses mentioned do not cause cold sores. The varicella-zoster virus is primarily associated with chickenpox and shingles. Human papillomavirus is linked to warts and certain types of cancers, particularly cervical cancer. Cytomegalovirus can cause a variety of illnesses but is not responsible for cold sores. Therefore, the specific association of HSV-1 with cold sores makes it the correct answer.