|Source: The Guardian UK|
Infectivity is the ability of a pathogen to establish infection. Infectiousness is the ability of an infection to spread from one person to another, by contact, inhalation or by way of a vector.
So while one may only need one particle of Ebola Virus to cause disease (infectivity), transmission (infectiousness) from person to person is not possible without direct contact of infected bodily secretions. In contrast, it takes more influenza viral particles to cause disease, yet these particles are more easily transmitted via inhalation of aerosoles or droplets.
The above has nothing to do with severity of disease as this is an entirely different matter. The fatality from Ebola is very high while fatality from influenza is low. Severity results from the host-pathogen interaction and can be mitigated by vaccines and treatments.
However, the burden of disease must also be considered. Given the prevalence of influenza worldwide, even with a fatality of < 1%, the estimated global burden is about 3-5 million cases of severe illness and about 250,000-500,000 deaths per year, as reported by the World Health Organization. This far outpaces the death toll from the recent Ebola outbreak.
The risk of contagion varies by infectivity, infectiousness (transmissibility) and burden of disease.