A lot of people think vitamins are useful to their health. In a generalization, taking a multivitamin would benefit a person's physical health. Vitamins are typical accompanied by a warning similar to that warning of exercise, "Check with your doctor before taking this." There is a subpopulation that could be injured by these vitamins. Most vitamins are sold being recognized as safe, although some nutritional supplements are restricted by age or may require a doctor's prescription. Society believes that vitamins as a whole are good for us, but for some subpopulations they are not safe to consume.
This brings us to the important discussion of suboptimal, optimal, and above optimal health. We could, for this discussion, simplify that optimal health would be considered 100% ideal health, perfect homeostasis. If a person is stressed, has poor nutrition, or lacks the necessary amount of sleep, the medical community would consider their health to be suboptimal. This is not a comprehensive list of reasons that health would be suboptimal.
Reducing stress, improving nutrition, or having a healthier amount of sleep would move most people toward the direction of optimal health. Note that there are already exceptions arriving. A person who does not have enough stress to be responsible may stop going to work or caring for their children. The stress of having to pay bills helps trigger a person to respect the need to work. The fear of a child not having a good life encourages a person to take care of the child. In regards to sleep, a person who has too much sleep may appear groggy. It is generally accepted that too much sleep is not a good thing.
Back to optimal health. People near optimal health, near the 100% perfect homeostasis, are typically thought to get "sick" less frequently than those with suboptimal health. These people are not immune to viral, bacterial, or fungal infections, but we consider a human's immune system near optimal health to be a better responder to a significant proportion of illnesses.
A person who is already stressed or short on sleep is likely to take longer to recover from a viral infection. This is considered common knowledge at this point. It does start to become debatable if a reduction in stress once the viral infection has already taken place will have a more than marginal effect on the person healing and recovering from the infection. I would expect that improving physical health during an illness would help remedy it more quickly.
Hydration is likely to be a measurable effect on reducing the symptoms from a viral infection. If your intake of fluids is reasonable, chances are you'll get better faster. The general thinking is that a physiological improvement during the infection may have a measurable effect.
People tend to not go to work when they're "sick." A part of this may be the person's desire to be courteous to co-workers or customers by not putting them in the position of potentially being infected if the sick person's illness is contagious. If a person is "sick" with something, being away from the stress of work could potentially improve their immune health. There is the assumption here that the person would be paid for sick days off, otherwise the stress of less income for the person's budget by missed days of work could actually increase their stress level.