system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do you doctors diagnose bacterial/viral illness?

I.E. Ebola etc.
gpt
Clinically, some bacterial/viral illnesses have very distinguishing features that give you a hunch towards what the disease may be caused from. Symptoms such as a certain rash, whether there is a fever, whether there is any pus, etc., point you in the direction of viral, bacterial, or even fungal infection.

If you think it's a bacteria, or fungus, you can sample it (from blood, urine, wound, etc) and allow it to grow or "culture". From there microbiologists can perform different tests on the bacteria that grow to determine which species it is and which antibiotics/antifungals are best for fighting them off (known as determining the bacteria's sensitivities). 

If you believe it's viral you often don't need to pursue it any further (especially if we're talking about a respiratory or sinus infection) as you often just treat the patient's symptoms  until the virus resolves. However, there are a few instances in which you need to determine the exact virus for treatment. In those cases, you would also similarly sample it (blood, CSF/fluid around your brain and spinal cord, cervix, etc.) and perform fancy biochemical tests to determine what type of viral strain it is.