Parasite initially costs 0 PWR to infect a device, and in general the PWR cost equals the number of devices currently infected by Parasite. When a device is infected, its firewalls are reduced by 1 at the start of each player turn; the device remains infected until all of its firewalls are broken. The PWR cost is spent when a device is infected, not when its firewalls are broken. Parasite has no cooldown.
Firewalls on multiple infected devices will be broken in the order they were originally infected, so if a daemon is installed on an infected device, it may be triggered before firewalls are broken on other infected devices. Parasite continues to break firewalls on devices even while Paradox is active, and even if Parasite is sold at a Server Terminal after the device is infected.
Parasite 2.0 is an upgraded version of Parasite, which infects mainframe devices to break 2 firewalls per turn at the start of each player turn. The device remains infected until all of its firewalls are broken. Parasite 2.0's PWR cost is equal to the number of devices currently infected by Parasite 2.0, plus 2. It has no cooldown. Parasite 2.0 can be bought from some Server Terminals for 500 credits.
It is possible to have both Parasite and Parasite 2.0 installed at the same time, in separate Incognita program slots. Devices infected by Parasite are not counted towards Parasite 2.0's PWR cost, and vice versa; it is not possible to infect the same device with both Parasite and Parasite 2.0.
Parasite's upside is that it can hack any number of firewalls for very little PWR.
Parasite's downside is that it is slow — unlike other breakers, it does not break firewalls immediately, instead you have to wait a number of turns equal to the number of firewalls before the device is hacked. Parasite can be sped up by breaking some firewalls using other programs, or Buster Chips, but the fact that it does not break firewalls instantly has far-reaching strategic consequences.
Because Parasite's PWR cost is low and does not depend on the number of firewalls, it can be more efficient than every other breaker. However, when used on more devices at once, Parasite has a higher PWR cost, so Parasite's PWR-efficiency depends on how many devices you hack with it, and how long each device stays infected. Still, Parasite generally uses less PWR than other starting breakers.
At low mission difficulties when most devices start with only 1 firewall, if no other devices are infected then you can hack one device per turn for free with Parasite until the firewalls rise at alarm level 2 (or alarm level 3 in Beginner mode). The alarm tracker steps up at the end of the enemy turn, before the start of your turn when Parasite breaks firewalls, so when playing with Parasite this effectively happens a turn sooner.
Parasite 2.0 is also generally a PWR-efficient breaker. It hacks faster than Parasite for a nominally higher PWR cost, but if you use it often then Parasite 2.0 can be more efficient overall, because devices finish hacking sooner, so fewer devices will be infected at the same time. Parasite 2.0 is often a good addition to Incognita.
If there are devices you don't need to hack quickly, then it may be good to infect them later after Parasite finishes hacking other devices, to cost less PWR. Once you have infected a device with Parasite or Parasite 2.0, it remains infected until all firewalls are hacked. This means it is not “free” to infect devices with high firewalls even for 0 PWR, because it raises the program's PWR cost for several turns.
Parasite slows you down most when you are waiting for it to hack Security Cameras. Unless playing with Internationale (or Oracle), to find cameras you usually need to peek at doors, often opening them to take a full peek. If you do this at the start of your turn, you may have to spend a full turn or longer just waiting for Parasite to hack the camera, so it is faster to open the door and peek at the end of the previous turn. However, opening the door can sometimes attract the attention of guards, so you will either need to ambush more often than usual, or plan your movement to have enough AP left to get into cover after peeking if necessary.
If you don't open the door for a full peek, you may wait a turn or two hacking one camera just to find that there is another camera on an opposite corner of the room.
As Parasite generally uses little PWR, it pairs well with almost any generator program. Despite its low PWR cost, Parasite even works well with Seed, because you can use Seed with a different program once per turn before using Parasite, or you can still use Parasite for 0 PWR once per turn when up to 4 devices are already infected.
If you get to higher mission difficulties with Parasite as your only breaker, the PWR cost and time taken to hack will become prohibitive, because you will usually need to infect several devices at once, and each device will be infected for several turns. When you choose Parasite as a starting program, you will want to add an instant breaker to your loadout quickly. An instant breaker lets you past cameras faster when they have higher firewalls, and can hack drones or Turrets in emergencies. Adding a second breaker also makes Parasite more PWR-efficient, as you will tend to have fewer devices infected and for shorter periods of time. Good options include Lockpick 2.0, Golem, Mercenary, or Dagger in low-PWR strategies; Parasite can also be used to bring firewalls down to the exact level required by Wrench.
When you infect a Security Camera while it is rebooting, such as an inactive camera that is coming online, its firewalls are broken at the start of your next turn, but the camera activates before then at the start of the enemy turn. Any agent in vision of the camera will be seen by it before Parasite breaks its firewalls, causing nearby guards to investigate.
When any devices are infected, beware of triggering Fractal, accessing the Primary Server Terminal in a Server Farm mission, using Taurus, or knocking out a Modded guard — these can install a daemon on an infected device, committing you to triggering that daemon, unless the device already has a different daemon installed.
When hacking devices with daemons, it is sometimes necessary to infect devices which have the same number of firewalls in a particular order, to mitigate the daemons' effects. Devices with Fractal, Rubiks and Castle should normally be infected after others, so the others will finish hacking before the daemon is triggered; devices with Echo should normally be infected before others, so that Echo won't recapture them immediately.