If you think about it, compared to humans, it is rare for your dog to get sick and your dog never seems to catch your human cold or flu. What’s the deal?

Germs Tend to be Species Specific

First, cold and flu causing germs tend to be species specific – more on that below. Harmful dog germs are usually only harmful to dogs and are not harmful to humans. Likewise, human germs won’t harm your dog. Kissing a dog is likely less harmful for a human than kissing another human as the human germs are more likely to be harmful to you than the dog germs.  Thus, when you get a cold, don’t worry about passing it to you dog (or cat).

Speaking of kissing, why do humans kiss?

Germs tend to be species specific because the cells of each species are different. Cells function to allow nutrients to come into the cell but keep out harmful things – like viruses and bacteria. The germs that infect a particular species have evolved to have a “key” that allows them to pick the “lock” that allows them to enter the cell, infect it and to use the cell to reproduce. That “key” usually only works on the “locks” of the cells of the specific species because the “locks” vary species-to-species. However, germs do mutate and evolve, so “cross-species transmission” can and does occur. New diseases that infect humans can come from other species. Think of bird flu or swine flu – these diseases were the result of mutations that allowed the viruses to jump species.

Related IFOD on Viruses and how they infect and replicate in cells Here

Dogs Don’t Ride Subways

Second, dog germs that cause illness aren’t passed dog-to-dog as much as humans pass their germs to each other. Dogs have much less contact with each other and there are a lot less of them (dogs aren’t packed into subways, stadiums and shopping malls with other dogs). Think of the other humans who live in your house and all the people you come in contact with during an average day. It can be hundreds or thousands of humans. Dogs, however, may only interact with a handful of dogs on a given day. Thus, dog germs aren’t passed around dogs to the same degree human germs are due to lack of contact. Note a sickness similar to a human cold that dogs get is called “kennel cough” because dogs tend to get it when they are in kennels where they have contact with a lot of other dogs.

Finally note that your dog can’t talk, so there may be days he’s not feeling well and you just don’t know it.