Play modes are split between five distinct modes, all based around the same core gameplay concept - survive.
In Normal, you need to avoid viruses while collecting DNA strands, which trail you similar to Snake. The more strands to collect, the longer your DNA tail becomes and the harder it gets to avoid viruses. If a virus hits a DNA strand, they go flying, forcing you to scramble and pick them up before they disappear. Power-ups periodically appear in the area. One gives you a missile-like projectile to destroy viruses. Another acts like a power pellet in Pac-Man.
Zoom 128 works in a similar way, but adds a tight camera zoom. As with Normal, your goal is to collect DNA, though here you only need to snag 128 in a set time. It sounds easy, but the tight camera adds a claustrophobic sense that, when combined with the fast-paced gameplay, can lead to a few anxious play moments. Progressive Mode is even more frantic. You can only move in a certain direction and really only have control over your speed.
Shooter is similar to Geometry Wars. Rather than collect DNA strands, you shoot viruses, but there's a catch. All of your shots are automatic and only happen every few seconds. It's an interesting twist on a common play type; every shot really does count. If you miss the timing by just a little, you're out of luck and need to avoid the growing viral swarms until the next shot loads. If you'd rather blast away at enemies, Horde Mode provides the same setup, but you get to pull the trigger.
All five modes are fun, but tend to drag after a while. Viral Survival isn't built for marathon sessions. It's a meant for short, action-packed sessions. Most of your motivation to keep playing will likely come from wanting to earn a top spot on the leaderboards or by earning achievements. I found both to be great motivators, but if climbing leaderboards or unlocking virtual brag tags aren't your thing, Viral Survival will only hold your interest for so long.