Hominidae Bridge is a tier 2 transmission exclusive to the Simian Flu DLC. It allows ape and human strains of the Flu transmit between species, increasing the chance of mutation and enabling the infection to spread from apes to humans (and vice versa). It can only be evolved after Simian Neuro-genesis is evolved.
This means that if there is a large amount of intelligent apes, evolving Hominidae Bridge will cause infected apes to infect the humans in their country, even if the normal infection that you started out with hasn't reached there yet. The same goes for apes, causing humans to increase infection rates between apes.
It does not increase infectivity, however. Using this transmission in conjunction with abilities in the Organised Travel tree allows for easy infection of any country, including island countries such as Greenland. If you infect all of the apes in the world, you can use this transmission to infect every country rather quickly, although not instantly. However, you must use transmissions to infect every person.