“Parasites are a lot like relationships,” my infectious disease professor said as he introduced the day’s topic.

“there are relationships where both parties benefit: that’s mutualism. Then sometimes one person benefits and the other is unaffected: that’s commensalism. And then you have those dangerous relationships when one partner benefits at the expense of the other: that’s parasitism.”

He then proceeded to show a series of photos of worst-case scenario parasite infections to the often audibly disgusted class. the take-home message: some types of relationships ought to be avoided at all costs.

Just like identifying a case of strep throat, it’s important that you know how to diagnose the kind of relationship you’re in. there are two different ways to make a diagnosis: identifying symptoms in the host (for example, a sore throat, fever and headache) or identifying the infectious agent (taking a strep test).

In relationships, I prefer the symptomatic approach because you won’t have to feel guilty for criticizing the flaws in your significant other. in general, it’s a bad thing to blame all the problems in a relationship on the other party. looking for symptoms in yourself, though, allows you to investigate the way the relationship is affecting you without judging the other party.

So, diagnosing relationships. the goal, of course, is a mutual relationship. both of you bring your best selves, your idiosyncrasies and your flaws, and for some reason you’re both happier together than apart. Synergy happens when your personalities combine to make something better than the sum of your separate selves. in humans, we have this relationship with our gut flora. Little bacteria live in your belly and help to digest food more efficiently. the bacteria get to live in a pool of acid, which they for some reason like, and you get to eat food, which you like.

You’ll know you’re in a mutual relationship when being with another person makes you want to be your best self. you may find yourself motivated to live the gospel, to be kinder, more patient or to think the best of others. in particular, you’ll feel inspired to serve your significant other without compulsion or guilt, but simply out of the pure love of your heart. If you’re in a relationship like this, hold onto it.

Next there’s the commensalistic relationship in which one member of the relationship benefits and the other is neither harmed nor benefited. this is like when a barnacle latches onto the outside of a boat or a whale or something. the whale is just fine, only that it’s covered in barnacles, and the barnacle has a place to stay.

Both in nature and in the dating world, commensal relationships are fairly rare because usually the barnacle eventually causes some harm or benefit to its host. For example, you might be dating someone to whom you’re fairly indifferent. He could break up with you, and you’d just go back to watching reruns of “American Idol” like nothing happened. same emotional payoff.

Eventually, you’re probably harmed in that you’re not dating someone you actually like, or you’ll get exhausted toting around some useless barnacle everywhere you go. On the other hand, perhaps you’re benefited because at least you no longer have to look like such a loser showing up to all your friends’ wedding receptions single. Either way, though, a commensal relationship will eventually manifest some signs of either mutualism or parasitism.

Finally, there’s the parasitic relationship. a parasite will suck the life out of you and leave you crippled. Loa loa is this horrible worm that likes to travel about a person’s insides, which is all well and good until it squirms across your eye. Not only does this make me want to lose my lunch, if the loa loa rubs up your eye enough times, you’ll eventually go completely blind. Cysticercosis is another disease that happens when you eat a taenia solium tapeworm in undercooked pork. Under certain circumstances, one of these things can form a cyst in your brain —destroying brain tissue to make space for itself.

You might be in a parasitic relationship if you have the following symptoms: you dread hearing from your girlfriend or boyfriend, and when you answer the phone you find that his or her needs just fester and take advantage of your kindness. Eventually you feel that your generosity has been leeched away, and all that’s left is resentment and weariness. you know you’re dating a parasite when you feel obligated to spend time with your boyfriend or girlfriend, and you leave feeling like the whole evening was about him or her. Yes, sometimes one party in a relationship will have more needs than the other, but hopefully that’s just temporary and you both get to have your needs met.

Once you realize his or her insatiable, parasitic neediness will be an ever-present drain on your relationship, you have to get out. Pry that parasite off. of course, both parties will be harmed when you two separate. When cysticercosis is removed, the little worm gets its just deserts (death by sanitation), and you still have a hole in your brain. If you’ve ever dated a parasite, you know that sometimes it’s immediately quite a bit more painful to pry him or her out of your life than to just endure her toilsome presence. but unlike brain tissue, the emotional/social/financial scars incurred by your ex-girlfriend or ex-boyfriend are reparable. Given a few months and several cartons of ice cream, you’ll be a whole person again. of course you might have some extra baggage, but hopefully you’re also stronger, wiser and less likely to get reinfected.