Brilliant question…we think this has already happened!
A virus called Nipah virus is spread by flying foxes to pigs and pigs can then pass it onto people. It is difficult to know for certain the exact details but in parts of Malaysia fruit trees were often grown right next to pig pens. The fruit bats as you can guess from their name love to eat fruit and they would come into the orchards each day (most species of fruit bats are active during the day and not at night) to feed, fruit bats are really messy eaters and lots of half eater fruit would drop onto the ground. Pigs were then eating any fruit that fell into their pen including fruit that had bat saliva or in some cases other waste products (you can imagine) on it.
The pigs became infected with Nipah virus, this was in 1999 and until then no one had ever seen or heard of this virus so at the time people didn’t realize how serious it was and unfortunately more than 100 people lost their lives in that outbreak. One of the difficulties faced when this outbreak occurred is that Nipah infection in pigs causes a coughing and wheezing (it is sometimes called “barking pig syndrome” or “one mile cough” as people said that you could hear the coughing pigs one mile away) but in humans there is not always a cough and many people develop a fever and headaches. Due to these differences people didn’t realise the link between the sick pigs and sick humans straightaway.
After that the authorities in Malaysia cracked down and more than 1 million pigs were put down to try to eliminate the virus. This seems to have worked and Malaysia hasn’t had any more outbreaks that we know about. However in Bangladesh Nipah has appeared again and again and it doesn’t seem to need pigs anymore to infect humans. In Bangladesh sap taken from Date Palm trees is a popular drink and the sap is harvested directly from the trees. The sap is harvested by making a cut in the tree and attached a bucket just underneath the cut to catch the sap, unfortunately the bats love the sap too and so bats come down and drink from the buckets mixing their saliva with the sap and sometimes even getting urine or faeces in the buckets.
Due to this Bangladesh has had quite a few Nipah outbreaks and most recently in 2011 the Bangladesh health authorities are now attempting to get the people that harvest the sap to use new stronger sealed buckets that will stop the bats getting at the sap and this seems to be working so far.