Bubonic plague has been detected in California and malaria at Heathrow Airport - two recent examples of how dangerous diseases normally considered to be under control in the west can re-emerge.
The bubonic plague was found within the past 10 days in Donner State Memorial Park, California. The malaria reported at Heathrow was caused by a rogue mosquito in luggage.
Researchers have warned that westerners are also particularly at risk from new disease-causing microbes that are resistant to current drugs.
The last century had seen tremendous progress in the fight against infectious diseases but there was no room for complacency, according to Prof Charles Penn from the Centre for Applied Microbiology and Research in Salisbury.
"We don't know what it would take to trigger an outbreak of the plague in the human population," stated Prof Penn.
Prof Penn and his colleague Prof Richard Wise from the Department of Medical Microbiology, City Hospital, Birmingham, were speaking at the British Association's science festival in Leicester about the current risks of infectious diseases.
The California outbreak involving the organism that causes the plague in humans was found in two squirrels and a cat, Prof Penn said.
The plague was less rare than might be assumed, however, as it was also reported a few times a year in other animals in America, including prairie dogs in the midwest.
Emerging viruses posed other dangers, according to Prof Wise. Five new types of simian immunodeficiency virus, which are similar to the HIV virus in humans, have been reported in primates in Africa, he explained.
"Nobody knows where HIV came from, but it could be a mutated virus from central Africa," said Prof Wise.
It was possible, then, that these new viruses might also be able to jump from animals to humans as some people were in contact with infected animals through eating "bush meat".
New infectious agents also posed a fresh challenge to our health aided by our own actions and technological advances.
The over-use and misuse of antibiotics had caused bugs that were super-resistant to most drugs currently available.