Health risks to humans

In the USA, cats and dogs are a factor in more than 86,000 falls each year. It has been estimated that around 2% of dog related injuries treated in UK hospitals are domestic accidents. The same study found that while dog involvement in road traffic accidents were difficult to quantify, dog associated road accidents involving injury more commonly involved two-wheeled vehicles. Toxocara canis eggs in dog feces can cause toxocariasis. In the United States, about 10,000 cases of Toxocara infection are reported in humans each year and almost 14% of the US population is infected. In Great Britain, 24% of soil samples taken from public parks contained Toxocara canis eggs.Untreated toxocariasis can cause retinal damage and decreased vision. Dog feces can also contain hookworms that cause cutaneous larva migrans in humans.
The incidence of dog bites, and especially fatal dog bites, is extremely rare in America considering the number of pet dogs in the country. Fatalities from dog bites occur in America at the rate of one per four million dogs. A Colorado study found that bites in children were less severe than bites in adults. The incidence of dog bites in the US is 12.9 per 10,000 inhabitants, but for boys aged 5 to 9 the incidence rate is 60.7 per 10,000. Moreover, children have a much higher chance to be bitten in the face or neck. Sharp claws with powerful muscles behind them can lacerate flesh in a scratch that can lead to serious infections.
In the UK between 2003 and 2004, there were 5,868 dog attacks on humans resulting in 5,770 working days lost in sick leave. There were 1,122 dog attack incidents reported by all New South Wales councils from 1 January 2010 to 31 March 2010. This number included harassment and bites by dogs on people and animals. The Staffordshire Bull Terrier was responsible for the largest number of attacks.

Info from: http://en.wikipedia.org/wiki/Dog