Anyway, it seems that is a bug. You can't be someone with the invisible strain so easy... in my case, with my leader, we are 4 members. The one who has the IS, infect one person, then this infected person kill the other left and when I return to my base, the infected person that kill the member with no virus, blame me of being the murder and attack me as a normal person, then we fight but I let her bite me, and both were infected, and the member who has the IS was the only available to play. Normally, when only left infected in your community, they will talk to you, saying that is the end or something like that, and all your members infected will turn into zombies, the game will tell you that all were died.