At the moment, you have 3 options to install VirtualBox using apt. Manually downloading / verifying / updating is not a good choice IMO.
4.3.36 = Oldest / Most Secure version from Jessie stable contrib:
Install:
sudo apt-get install virtualbox linux-headers-$(uname -r)
Guest Additions (if needed):
sudo apt-get install virtualbox-guest-x11
5.0.14 = Newer / Less Secure version from Jessie-backports:
Add to /etc/apt/sources.list:
deb http://http.debian.net/debian jessie-backports main contrib
Install:
sudo apt-get -t jessie-backports install virtualbox
Guest Additions (if needed):
sudo apt-get -t jessie-backports install virtualbox-guest-x11
5.0.14 = Latest / Least (?) Secure version from Oracle repository:
Add to /etc/apt/sources.list:
deb http://download.virtualbox.org/virtualbox/debian jessie contrib
Add Oracle key to apt-get keyring:
wget -q https://www.virtualbox.org/download/oracle_vbox.asc -O- | sudo apt-key add -
Check fingerprint:
7B0F AB3A 13B9 0743 5925 D9C9 5442 2A4B 98AB 5139 Oracle Corporation (VirtualBox archive signing key) <info@virtualbox.org>
Install:
sudo apt-get update
sudo apt-get install virtualbox-5.0
Guest Additions ISO is included in the package. (Strange since repository is contrib only). Insert ISO in guest to install.
EDIT: virtualbox-guest-x11 should be used over virtualbox-guest-additions-iso in debian. Thanks @TNT_BOM_BOM