The Supreme Court has stated that the Preamble is meant to bear witness to the fact that the Constitution emanates from the People and not from the states.(1) It does not confer any specific powers upon the government, but acts to convey the scope and nature of the document overall.(2) So, from the best of my understanding, this line on its own doesn't give Congress any power whatsoever. Article I, Section 8 of the Constitution grants these powers to Congress:
Punishing piracy and crimes on the high seas;
Declaring war, issuing letters of marque and reprisal;
Raising and maintaining armies (with a two-year limitation on funds);
Raising and maintaining a navy;
Deciding how land and naval forces will be governed and regulated;
Calling on the militia to execute the laws of the Union, suppress insurrections, and repel invasions;
Providing for the organization, arming, and disciplining the militia, and governing any part of the militia that might be employed in the service of the Union, with certain aspects reserved for the individual states.
While I'm not a Constitutional scholar or a lawyer, I can't rattle off specific court cases that address broad interpretations of Congressional powers related to defense, I think that extending Congress' Section 8 powers to apply to a health threat is sketchy at best. I'm not trying to piss in your Cheerios; it's just that I can't find anything that would come close to supporting such an interpretation. If you can find anything (legal precedents, rulings, etc.), I'd be very interested.
The constitutional meaning of "welfare" is, to the best of my knowledge, health, happiness, or prosperity; well-being. Again, the Preamble confers no power in and of itself. Promoting the general welfare is a broad concept, but we can look to Article I, Section 8, which states the powers of Congress. What we see there are things like regulating commerce, establishing post offices, and coining money. There is a power granted to Congress involving the progress of science, but this is limited to securing exclusive rights to discoveries for the discoverers or developers for a limited time. There is absolutely nothing granting Congress the power to regulate individual freedoms. Again, I'd be happy to be pointed to court cases addressing the "general welfare" concept.
The 9th Amendment states that just because a right (e.g. the right to consume tobacco or the right to not be forced to consume tobacco) is not specifically addressed doesn't mean the government can step in and take control of the issue. The 10th Amendment ensures that any and all powers not delegated to the 3 branches of government falls to the States an the People. If the power isn't specifically given, it cannot be exercised.
FOOTNOTES:
(1) McCulloch v. Maryland, 17 U.S. (4 Wheat.) 316, 403 (1819) Chisholm v. Georgia, 2 U.S. (2 Dall.) 419, 471 (1793); Martin v. Hunter's Lessee, 14 U.S. (1 Wheat.) 304, 324 (1816), and that it was made for, and is binding only in, the United States of America. Downes v. Bidwell, 182 U.S. 244, 251 (1901); In re Ross, 140 U.S. 453, 464 (1891)
(2) Jacobson v. Massachusetts, 197 U.S. 11, 22 (1905)