One of the world’s stealthiest pieces of malware infected more than 4.5 million PCs in just three months, making it possible for its authors to force keyloggers, adware, and other malicious programs on the compromised machines at any time. The latest TDL-4 version of the rootkit, which is used as a persistent backdoor to install other types of malware, infected 4.52 million machines in the first three months of this year, according to a detailed technical analysis published Wednesday by antivirus firm Kaspersky Lab.