The "Immortality virus" was the tenative title of a mutagen being developed by Dr. Alex Wesker, under the authority of Dr. Oswell E. Spencer, Earl Spencer. While the virus' completion was announced, Spencer never received a sample, and so concluded that his trusted friend had finally betrayed him.
In reality, Dr. Wesker failed to develop this virus due to the difficulties she had on the experiment, and had grown disloyal to Spencer as well. Instead Dr. Wesker switched to create something else, while making various fake reports to Spencer in order to pacify him. Once she finished her goals, she and her subordinates fled without leaving a trace.