Why must chicken be cooked through when steak can be rare?

Because of where the bacteria are. In a whole cut of beef, contamination is on the surface. In poultry, it can be throughout the meat.

Bacteria arrive on meat during slaughter and processing, and they land on the outside. A whole muscle — a steak, a joint of beef, a lamb chop — has an intact interior that is effectively sterile. Searing the outside kills what is there, so the inside can be left rare without the same risk. This is why a rare steak is normal and a rare beefburger is not: mincing distributes surface bacteria throughout the meat, so mince must be cooked through.

Poultry is different for two reasons. Salmonella and Campylobacter can be present in the bird's tissue and not only on its surface, and poultry's structure — with skin, joints and cavities — makes surface contamination far harder to control. So the whole piece must reach a safe temperature.

Pork was historically treated like poultry because of Trichinella, a parasite. Modern farming has made that risk very rare in many countries, and official guidance in several has been revised to allow pork cooked to a lower temperature with a rest, rather than well done.

The reliable tool is a probe thermometer, not colour or clear juices — both are unreliable indicators. Cooking is a combination of temperature and time held at that temperature, which is why sous vide can pasteurise at lower temperatures than fast roasting.

Safe temperatures differ between countries and are periodically revised, so follow your national food safety agency's current guidance rather than a figure remembered from elsewhere.