Leftovers can be kept for three to four days in the refrigerator. Be sure to eat them within that time. After that the the risk of food poisoning increases. If you don’t think you’ll be able to eat leftovers within four days, freeze them immediately.

Food poisoning — also called foodborne illness — is caused by harmful organisms, such as bacteria in contaminated food. Because bacteria typically don’t change the taste, smell or look of food, you can’t tell whether a food is dangerous to eat. So if you’re in doubt about a food’s safety, it’s best to throw it out.

Fortunately, most cases of food poisoning can be prevented with proper food handling. To practice food safety, quickly refrigerate perishable foods, such as meat, poultry, fish, dairy and eggs — don’t let them sit more than two hours at typical room temperature or more than one hour at temperatures above 90 F (32 C).

Uncooked foods, such as cold salads or sandwiches, also should be eaten or refrigerated promptly. Your goal is to minimize the time a food is in the “danger zone” — between 40 and 140 F (4 and 60 C) — when bacteria can quickly multiply.

When you’re ready to eat leftovers, reheat them on the stove, in the oven or in the microwave until the internal temperature reaches 165 F (74 C). Because they may not get hot enough, slow cookers and chafing dishes aren’t recommended for reheating leftovers.

Answer from Mayoclinic.com