How do they know that? Bit of a sweeping statement.
I had a virus recently that made me very ill for nearly 2 days. I have never been so ILL in my life. It was not Swine Flu or Winter Vomiting Bug and a week after I still do not know what struck me down but it also affected others I know but have not had contact with for weeks.
I do have it on good authority it was a VIRUS. These little bugs mutate and can kill us too once they find a way around our defences.
There is no threat to humans or other animals. 'Not at this time'