Skin infections can be caused by various microorganisms. The correct answer is option 4, which states that all the mentioned microorganisms can cause skin infections.

I. Protozoa: Although not as common as bacteria and viruses, certain protozoa can cause skin infections. Examples include giardiasis, a parasitic infection that can result in skin rashes and itching.

II. Bacteria: Bacterial infections are a common cause of skin infections. Staphylococcus and streptococcus bacteria are often responsible for conditions like impetigo, cellulitis, and boils.

III. Virus: Certain viruses can also lead to skin infections. Examples include herpes simplex virus (causing cold sores and genital herpes), varicella-zoster virus (causing chickenpox and shingles), and human papillomavirus (causing warts).

Therefore, all three options (I. Protozoa, II. Bacteria, III. Virus) are correct, making option 4 the correct answer.