Eat good, nutritious food.
Make sure you aren't deficient in any mins and vits, including Vit C, Vit D and all the ones that work in synergy with those vits, and which benefit the immune system.
Get enough exercise.
Avoid junk food and excessive alcohol.
Keep away from people who are coughing and spluttering.
Don't touch your face with your hands if you have been in public areas or touched public items (such as water fountains and door handles)
Wash your hands regularly, thoroughly and using soap, after being out in public.
And I have noticed that I often used to come down with a virus around 3 days after a severe hypo. So I assume that the hypo caused my immune system to dip leaving me open to the virus. Now I have improved my glycaemic control, that pattern has stopped happening.
Hope that helps.