Tick bites are one of the risks that come with letting your dog outside. High grass, forests and even running water can all be places where a dog will pick up ticks. While some tick bites may be harmless, even if left untreated, others may cause a variety of unpleasant side effects.
Lyme disease is a particularly vicious condition that can affect dogs bitten by deer ticks. Deer ticks are commonly found in heavily wooded areas. Lyme disease is caused by bacteria inside the deer tick, and the tick has to be attached to the dog for a minimum of 48 hours before the disease can be fully communicated. If left untreated the dog can develop full on Lyme disease, which is much harder to cure once it's been allowed to progress fully. Early treatment of the bite can make the disease easier to cure, however.
If a tick bite is left untreated it can cause a dog to have an allergic reaction. An allergy is when the immune system creates antibodies and inflames in a way that's meant to help protect the body and to fight off infection, but the body begins attacking itself rather than some outside threat. With proper treatment on a tick bite an allergic reaction can be suppressed so that the animal is safe. However, if left untreated severe allergic reactions can sometimes lead to a dog's death.
Even though a tick bite is relatively small, it's still a wound. If that wound is left untreated it can result in an infection. This is particularly true if the dog in question spends a lot of time outside where there are numerous bacteria and viruses that would be easily able to slip through such a small tear in the dog's skin. For this reason all tick bites should be treated as soon as they're found, often with an antibiotic cream to ensure that infection is minimised as soon as possible.