Fishes have a more or less smooth, flexible skin dotted with various kinds of glands, both unicellular and multicellular.

A tissue, organ or organism that is made up of many cells is said to be multicellular. Animals, plants, and fungi are multicellular organisms and often, there is specialization of different cells for various functions.

A living thing, whether made of one cell (like bacteria) or many cells (like a human), is called an organism.

Multicellular organism, an organism composed of many cells, which are to varying degrees integrated and independent.

The first evidence of multicellularity is from cyanobacteria-like organisms that lived 3–3.5 billion years ago.

Unicellular organisms are made up of only one cell that carries out all of the functions needed by the organism, while multicellular organisms use many different cells to function. Unicellular organisms include bacteria, protists, and yeast.

Bacterial cells are fundamentally different to the cells of multicellular animals such as humans. They are far smaller, with less internal organisation and no nucleus (they have DNA but it is not packaged safely within a membrane).

Yeast are a polyphyletic group of species within the Kingdom Fungi. They are predominantly unicellular, although many yeasts are known to switch between unicellular and multicellular lifestyles depending on environmental factors, so we classify them as facultatively multicellular (see Glossary).

Prokaryotes (PRO-kaer-ee-oats) are individualists. These organisms are small and single-celled. Eukaryotic cells are generally bigger — up to 10 times bigger, on average, than prokaryotes. Their cells also hold much more DNA than prokaryotic cells do.

Protozoa (pronounced: pro-toe-ZO-uh) are one-celled organisms, like bacteria. But they are bigger than bacteria and contain a nucleus and other cell structures, making them more similar to plant and animal cells.

Nor do viruses have cells: they’re very small, much smaller than the cells of living things, and are basically just packages of nucleic acid and protein. Still, viruses have some important features in common with cell-based life.

Viruses are found wherever there is life and have probably existed since living cells first evolved. The origin of viruses is unclear because they do not form fossils, so molecular techniques are used to investigate how they arose.

Viruses cause familiar infectious diseases such as the common cold, flu and warts. They also cause severe illnesses such as HIV/AIDS, Ebola, and COVID-19. Viruses are like hijackers. They invade living, normal cells and use those cells to multiply and produce other viruses like themselves.