How a computer can infect others devices on the same network?

Question

As I said in this post, my father is very careless about online security. Besides what I already said on that post, yesterday I saw in his android that the permission to install of third-party apps are enabled, and he confirmed that he disabled this protection to install something that he even remember what is it (I think this was recently, because he did not want to answer anything about the app's nature or when he did this). I have already talk to him about his negligence, but nothing appears to has effect.

I'm worried because my mother accesses her bank account here (in her own PC, not in my father's PC). Some members in this forum recommended to me create a separate network for me and my mother's devices, but I am have difficult to find information about how do this and I don't have knowledge about networks.

I would like to know:

1. How likely is to a virus (or any malicious program/app) spread through devices on the same network?

2. What type of malicious application can do that?

3. It is possible for me to identify if that occurs without doing a long search?

I'm accepting other suggestions about what I can do.

Obs: All devices here are windows or android.

Answer 1

Many different types of malware can spread from device to device on the network. Wannacry was a very famous recent one.

Firewalls that block incoming connections is the most basic protection against this. You should have no reason to accept incoming connections. Make sure your Windows firewalls are set up and that you are running anti-virus programs.

Android phones do not run services that you can connect to, so they do not need firewalls.

From your other post, because your router cannot provide a second network, what you need to do is to get one that does. It's just that simple.