The thermometer probe goes into raw meat and carries the bacteria on its surface. When the same probe is inserted into cooked meat without being washed, it deposits those bacteria directly in the center; a single action undoes the safety achieved through cooking. Wash the probe with hot, soapy water and dry it after every measurement. The rule that applies to cutting boards applies to thermometers, too.
When people think of cross-contamination in the kitchen, they think of cutting boards, knives, and plates; thermometers rarely make the list. Yet the probe goes deeper than all those tools. The metal rod that enters raw meat picks up bacteria on its surface and continues carrying them when it comes out. It looks clean when wiped, because bacteria are invisible and leave no trace. Looking clean does not mean being clean; the residue on the probe can only be removed by washing. Because the probe is thin, grease and meat juices on it are easy to miss.
The real problem comes with the second measurement. Heat makes the surface of cooked meat safe, while its center is the most protected part. When an unwashed probe enters that center, it deposits the bacteria it picked up from raw meat directly there. If the meat will not be reheated after this, the bacteria remain there and begin to multiply. All the safety gained through cooking is undone by a two-second action. The same issue applies in exactly the same way to fish, poultry, and ground meat.
The cleaning method is simple. Wash the probe with hot, soapy water after every measurement, rinse it, and dry it with a clean paper towel. Dipping it into water boiling in a pot also works, but soapy water is more reliable because it dissolves grease. Wiping it with a towel or kitchen cloth is not enough; the cloth picks up the residue and transfers it to another surface the next time it is used. Do not submerge the body in water; wash only the probe and keep the connection point dry. Do not put the washed probe into its case while wet; moisture left inside the case can carry over to the next use.
The order of measurements also reduces the risk. If there are different meats on the same tray, start with the lowest-risk one and leave the piece that is still raw for last. If you need to measure several spots in a piece of meat, take the measurements in succession without switching to another food in between. If you take a final measurement after cooking, the probe must have been washed. The same rule applies to tongs, forks, and knives; a tool that touched raw food must not touch cooked food.
Thermometers with a probe that stays in the food have a slightly different setup. That probe remains inside during cooking and becomes safe along with the food; the issue is when it is inserted and removed. Wash your hands after inserting the probe into raw meat, and do not leave the cable loose on the counter. When cooking is done, remove the probe and do not put it back into the food. An infrared thermometer does not have this issue, but it measures only surface temperature; it does not replace measuring the internal temperature. Do not leave the thermometer on the counter after cooking; wash it and put it away in its place.