Micro-organisms that infect humans are of several types, in descending order of their size being: fungi (yeasts, e.g. Candida), bacteria (e.g. Toxoplasma Gondii) and viruses (e.g. HIV), some of which may lead to infections with maternal-foetal transmission.

There are many differences between viruses and bacteria. They relate both in their dimensions, the multiplication methods, the life span, and in the resistance to treatment. Compared to viruses bacteria are more complex, bacteria being microorganisms that have in the genome both DNA and RNA, synthesize their own proteins and are covered by a rigid cell wall and a thin cell membrane that surrounds Cytoplasm.