Tiny organisms like bacteria and viruses don't do very well on their own. While some may be able to survive for a time, they need host species in order to thrive and reproduce in great numbers. These tiny invaders may be spread through the air, on dishes, or other surfaces in our homes or outdoors, including on feces or urine. While children eventually learn (at least we can hope!) to wash their hands and not put their fingers in their mouths, our dogs will naturally smell, sniff, and investigate with nose and tongue. You know how colds can spread through classrooms or on airline flights — dogs can be exposed at dog parks, in training class, kennels, or shows.

Respiratory Spread

The canine body is designed to keep invaders out of the respiratory tract. Along with its fabulous scenting cells, the nose has cells with cilia (thin strand-like projections) that line most of the respiratory tract. The cilia act to nab any debris — from dust to bacteria — that floats by them on the air currents. A thin mucus coating helps to trap anything that comes by. These cilia then use a concerted wavelike action to move that debris back up and out. When your dog coughs or sneezes, he is removing debris that the cilia caught.

Unfortunately, the cilia are very sensitive to the environment. They can be damaged in air that is very dry or conversely very humid, by secondhand smoke, or by other fumes in the air that passes over them. Some viruses and bacteria attack these cells directly and lower their defensive capabilities.

When a dog with a respiratory infection coughs or sneezes, she sends tiny droplets of infectious material into the air. If your dog breathes those in and they manage to get by her cilia, she has a good chance of being infected herself. Cells that fight infection, such as neutrophils and lymphocytes will then try to capture the infectious material and destroy it or at least neutralize it by wrapping it up in a defender cell.

Oral Spread

As you know, when your dog sniffs something interesting, he will usually lick it or even take a small taste. If he steps in something smelly, he will carefully lick his feet clean afterward. This practice can lead to the oral spread of diseases. For example, if he licks some material containing parvo virus (such as stool passed by a sick dog), he then takes the virus directly into his system. Many viruses try to get integrated into cells that divide rapidly as this helps them to spread quickly. Intestinal cells, with their very short turnover time, are ideal hosts for viruses.

Dogs may also drink in infectious agents from contaminated water. Giardia is a parasite that can be picked up from water.

Spreading via the Blood

Just like in the horror shows, some diseases can be spread by direct injection into the bloodstream. It is not space aliens doing the injecting, however, but insects such as mosquitoes, along with parasites like fleas and ticks that put diseases into our dogs as they take their blood meals out. When a mosquito bites a dog that is infected with heartworm, the infectious larvae may be passed into the next dog the mosquito bites. Deer ticks are renowned for spreading Lyme disease. Open sores or wounds can admit diseases into the body as well. Once the infectious agent is in the blood, it can travel throughout the body to the desired final location.