Is there any risk of illness from eating a slice of pizza?

Yes. Frozen pizzas may be made of raw ingredients contaminated with E. coli O157:H7 and salmonella as a result of the way the ingredients were farmed, harvested, and processed. These ingredients include, but aren't limited to, dairy products (cheese), meat (beef, pepperoni and sausage), seafood (shrimp and anchovies), and tomatoes, onions, mushrooms, and  peppers.  Freezing inactivates these pathogenic bacteria but doesn't destroy them. Fortunately, baking a pizza as directed does.
A delivered pizza may contain the same microorganisms if it hasn't been thoroughly baked.  Signs of thorough baking include a crispy crust, browned cheese, and hot toppings. If the pizza doesn't appear to have been thoroughly cooked, reheat it to a temperature of 165°F.
If the pizza appears done but arrives lukewarm, reheat it to 165°F. This will destroy any bacterial growth that might have occurred while the pizza cooled in transport. Depending on how--and how long-- it is stored, leftover pizza may be subject to spoilage bacterial and mold growth.  Pizzas may also contain common food allergens, including dairy products, soy, gluten, and seafood.
 Fortunately, proper storage, handling and preparation of pizza eliminate most risk of illness.
 Source(s):
Schmidt & Clark "Jeno's & Totino's Frozen Pizza Recall Lawsuit"
Healthgoods.com "Food Storage Guidelines for Consumers"