The magic answer...
You probably assumed I was going to say that... because, as you know, it always depends!
I recommend asking yourself a slew of questions:
1. What are your symptoms?
Without hesitation, do not train if you have diarrhea, are vomiting, or have a temperature. No one wants the squirts mid-squat. Literally no one.
The questionable symptoms will likely be headache, sore throat, and sneezing. These will all depend on severity and if it's paired with anything else. For example, if you're hacking up a storm, it's probably best you don't workout, but if you just woke up with a slightly scratchy throat, maybe you could get by and just reduce the workout intensity.
2. Have you trained with your current symptoms before? How did you feel after?
3. Do you feel like you could benefit from moving, but maybe take it down a notch?
4. Do you feel a rest day would be more beneficial?
5. Would sleep, water, and food help? If so, what if you try training later instead?
Again, your responses to these questions will help you determine how you could move forward.
If I have an in-person client who notifies me that they aren't feeling tip top, but are willing to try the workout, this is what I do:
1. Plan for a less intense workout (reduce volume, increase rest times, etc.).
2. Take them through their warm-up and see how they feel after the warm-up. Still feel shitty? It's a no-go. Feeling okay? We'll keep going and I'll check in frequently to see if things have gotten better, worse, or the same.
3. Listen to the client. I can't tell them how they feel. I can only perceive how they're acting and responding. They may feel much worse than they look, or vice versa. Communication from both parties is key.
Ultimately, always consult with your doctor if you're unsure.