It is NOT fact it cannot be spread by aerosol: it mutates, and its brethren can (Reston Virus, which may be on the loose in Virginia: it's got very low pathogenicity in humans and may even be the basis for an innoculation). Experimental research HAS shown airborn transmission from pigs to and between primates can happen, see below. As at this point, it has not jumped the air gap, but that does not mean to say it cannot, will not, or is not doing so now.

The Spanish nurse affirms she took every precaution against the known normal pathology. As she was infected none the less, then the pathology has changed, and the pressure is on to discover why. In the mean time, those involved must be ultra cautious: there is, for example, an obvious discrepancy between the air control in the Royal Free setup and the "let's swan around in the open" in Madrid. Watching them, they only double-glove, for example, which is one layer too few: one for the patient, one for the environment, and one for you.

See:
Johnson E, Jaax N, White J, Jahrling P (Aug 1995). "Lethal experimental infections of rhesus monkeys by aerosolized Ebola virus". International journal of experimental pathology 76 (4): 227–236. ISSN 0959-9673. PMC 1997182. PMID 7547435.
Weingartl HM, Embury-Hyatt C, Nfon C, Leung A, Smith G, Kobinger G (2012). "Transmission of Ebola virus from pigs to non-human primates". Sci Rep 2: 811. doi:10.1038/srep00811. PMC 3498927. PMID 23155478.