The movement of large numbers of animals across long distances also brings together many diseases that were previously confined to a species or a place (in some cases both). One reason that captive bred birds cannot be re-introduced into wild populations is because birds in the pet trade have been exposed to a variety of diseases that could wipe out natural populations that have no immunity to them.
With the growth of the wildlife trade, the number of diseases spread by animals to humans, Zoonoses, has also increased. Animals and people are now vulnerable to new diseases, parasites, viruses and funguses that they were not previously exposed to.
In 2003, Severe Acute Respiratory Syndrome (SARS) brought Hong Kong to a standstill. The disease was brought into Hong Kong by an infected Mainland doctor and almost 300 people in Hong Kong died from the virus shortly after.
The source of the virus remains unknown - it is believed to have originated in the horseshoe bat - but at the time the virus was found in 5 Palm Civets (Paguma larvata) and a raccoon dog in a wildlife meat market in Guangdong, South China. A Chinese Ferret Badger from the market was found to carry the antibodies from the disease. Scientists speculate that the virus jumped from these animals to humans and were subsequently spread by wild animal traders to their patron.
There are many strains of Avian flu but most do not cause death in humans. However, in 2001 and 2002, a particular strain, H5N1, caused severe disease and death in humans. Most of the people who died due to bird flu had close contact with poultry. Wild birds were also found to carry and die from the illness. Due to the highly intensive faming methods, millions of chickens were killed to stop the spread of the disease. One million chickens were slaughtered each time when H5N1 was detected in Hong Kong in 2001 and 2002.
Salmonella is a bacteria that is carried in the intestines of humans and animals. Though it can also be contracted in many ways (e.g. eating unhygienically prepared food at a Dai Pai Dong), children have also fallen very sick after handling reptiles that carry Salmonella. Salmonella infections are especially serious in babies, children and people with weak immune systems. Most people have symptoms similar to having a bad case of food poisoning- diarrhea, fever and stomach cramps but in serious cases, can lead to septicaemia (blood poisoning) and meningitis (swelling of the brain membrane).
In the 1970s, salmonella was consistently found in small pet turtles, leading to a ban in the trade of turtles smaller than four inches in size in the US. All reptiles carry Salmonella and shed them in their feces. Treatment with antibiotics has not been successful and there are concerns that this will only increase resistance. The Communicable Disease Centre in the United States has seen a sharp increase in Salmonella cases with the growing popularity of reptiles as pets. Recognising the risks to children it is not recommended that reptiles are not kept as classroom pets.