virus
/ VY-rus /
A virus is far smaller and far simpler than a cell — so simple that on its own it can do almost nothing. Picture a microscopic package: a bit of genetic instructions sealed inside a protein shell, and nothing more. Outside of a living cell it just drifts, inert, like a seed that will never sprout unless it lands in exactly the right soil. The 'right soil' for a virus is the inside of a living cell, which it hijacks to make copies of itself.
Structurally, a virus is genetic material (DNA or RNA, depending on the virus) wrapped in a protein coat called a capsid; some viruses add an outer membrane envelope stolen from a previous host cell. A virus has no cytoplasm, no ribosomes, no metabolism, and cannot generate energy or build its own proteins. Because it lacks the machinery to reproduce by itself, it must inject or deliver its genetic material into a host cell and borrow that cell's machinery to manufacture more virus particles, which then burst out to find new cells.
Viruses sit on the boundary of what we call 'alive.' They evolve, they carry genes, and they can devastate populations — yet they cannot grow, cannot maintain themselves, and cannot reproduce without a host, so most biologists do not count them as living in the way a cell is. This is the key distinction in this field: a bacterium is a living cell; a virus is not a cell at all. That difference also explains why antibiotics, which target bacterial cell structures, do nothing against viruses like the common cold or flu.
The flu virus is just eight short strands of RNA wrapped in protein and a borrowed membrane. It cannot make a single copy of itself in a test tube of nutrients — but slip it into the cells lining your nose and throat, and those cells will obediently churn out thousands of new flu viruses.
Inert in a test tube, unstoppable inside a host cell — the defining paradox of a virus.
Viruses are not cells and are not killed by antibiotics, which only target bacterial cell structures. Taking antibiotics for a viral cold does nothing useful and helps breed resistance.