Complex sentence: The FBI estimated that more than four million computers in over 100 countries were infected by this malware, which is the DNSChanger family; this included the RSPlug Trojan horse, which spawned a number of variants and infected many Macs.
Split into simpler sentences: The FBI estimated that more than four million computers in over 100 countries were infected by DNSChanger. One variant of DNSChanger was the RSPlug Trojan horse, which spawned a number of other variants and infected many Macs.