• Eat a healthy diet
  • Eat a diet high in fruits, vegetables, and whole grains, and low in saturated fat.
  • Eat lean protein. The antibodies that help fight disease are actually made of protein.
  • Keep your vitamin D level at 50 nmol/L. New research suggests vitamin D could boost immune response.
  • Cook with olive and canola oils. These contain healthy fats, which act as a lubricant for cells. This lubricant improves flexibility and communication between the cells, which promotes immune function.
  • No smoking, drink alcohol only in moderation.
    Excessive alcohol consumption is toxic to immune system cells called dendritic cells.
• Exercise regularly.
  During moderate exercise the activity of Natural Killer (NK) immune cells is enhanced. See below.
• Maintain a healthy weight.
• Control your blood pressure.
• Get adequate sleep.
• Take steps to avoid infection, such as washing your hands frequently and cooking meats thoroughly.
• Get regular medical screening tests for people in your age group and risk category.

The Society for General Microbiology (SGM) article "Couch Potato Or Elite Athlete? A Happy Medium Keeps Colds At Bay!" in 2012 says that "While regular moderate exercise can reduce the risk of catching cold-like infections, prolonged strenuous exercise, such as marathons, can make an individual more susceptible."

During moderate exercise the activity of Natural Killer (NK) immune cells is enhanced.

Links:
Google Search Boost your Immune system
How to boost your immune system : Harvard Health Publications
Germs - Pathogens (Viruses, Bacteria, Protozoans, Fungi)
Colds
  Flu