It is a big jump from increasing fish oil intake to catching a cold in three days.
Colds are usually transmitted through contact with your fingers and the mucus membranes of your eyes and nose.
The best way to avoid colds and flus is to practice advanced hygiene - cleaning your hands and performing facial dips or similar to keep your nasal passageways clean.
Large doses of fish can in theory supress the immune system (described in the Omega RX zone) but this is only theory.)
In practice fish oil will more likely stimulate your immune system by increasing the production of good eicosanoids.
If I had MS I would be taking large doses of pharmaceutical grade fish oil and following a zone diet.
If you haven't read them already I suggest you read my notes on MS on this forum.
Of course ultimately you have to make your own decisions and trust your own innate wisdom.