My understanding is that they can't. And for the most part you can't get sick from them, either.
From the CDC:
Diseases from Dogs
Important Tip!
Many germs can be passed to people from dog bites. Learn more about how to prevent dog bites from the Humane Society of the United States.
Although dogs can pass germs to people, you are not likely to get sick from touching or owning dogs. To best protect yourself from getting sick, thoroughly wash your hands with running water and soap after contact with dogs, dog saliva, or dog feces (stool).
Dogs can carry a variety of germs that can make people sick. Some of these germs are common and some are rare. For example, puppies may pass the bacterium Campylobacter in their feces (stool). This germs can cause diarrhea in people. Less often, dogs in urban or rural areas can carry the bacterium Leptospira (lep-TO-spy-ruh). This germ causes the disease leptospirosis (lep-to-spi-roh-sis) in people and animals. Dogs can also carry rabies, a deadly viral disease. Rabies from dogs is rare in the United States.
Some people are more likely than others to get diseases from dogs. A person's age and health status may affect his or her immune system, increasing the chances of getting sick. People who are more likely to get diseases from dogs include infants, children younger than 5 years old, organ transplant patients, people with HIV/AIDS, and people being treated for cancer. Special advice is available for people who are at greater risk than others of getting diseases from animals.
http://www.cdc.gov/healthypets/animals/dogs.htm