You can explain that traditional viruses are less common on Linux. Rootkits and privilege escalation exploits are more likely. One strategy might be to say tha

You can explain that traditional viruses are less common on Linux. Rootkits and privilege escalation exploits are more likely. One strategy might be to say that Linux systems are not sheltered from any threat (saying otherwise might lead the other person to consider you as a biased Linux fan), but instead to explain that the threats and consequently the defensive tools to adopt are different. So, instead of an AV, I would recommend 1) an host-based intrusion detection system (HIDS), like OSSEC or Tripwire, and 2) a kernel patched with Grsecurity. The HIDS will send you an alert every time an important file (like /etc/shadow) or a new binary / package is installed or modified. Grsecurity will offer multiple very efficient protection mechanisms against privilege escalation exploits (among other things). For the MacBooks, I don't know.