Malware and its devices
The first is a Virus, it copies itself and spreads, but in order for it to spread it must have contact with other devices. These could be in a link or sent through email or text.
Second is a Trojan horse, they seem like the real thing but as soon as you accept it opens the flood gates for other malware.
The last of out three is spy ware, it secretly obtains personal information.
These three are bad news, so to stay safe use your antivirus software, firewall but most of all common sense.